diff --git a/src/cmd/go/get.go b/src/cmd/go/get.go
index 90ac832a4bfdf..99d11ac2fa388 100644
--- a/src/cmd/go/get.go
+++ b/src/cmd/go/get.go
@@ -89,8 +89,12 @@ func runGet(cmd *Command, args []string) {
 
 	// Phase 1.  Download/update.
 	var stk importStack
+	mode := 0
+	if *getT {
+		mode |= getTestDeps
+	}
 	for _, arg := range downloadPaths(args) {
-		download(arg, nil, &stk, *getT)
+		download(arg, nil, &stk, mode)
 	}
 	exitIfErrors()
 
@@ -163,15 +167,15 @@ var downloadRootCache = map[string]bool{}
 
 // download runs the download half of the get command
 // for the package named by the argument.
-func download(arg string, parent *Package, stk *importStack, getTestDeps bool) {
-	load := func(path string) *Package {
+func download(arg string, parent *Package, stk *importStack, mode int) {
+	load := func(path string, mode int) *Package {
 		if parent == nil {
 			return loadPackage(path, stk)
 		}
-		return loadImport(path, parent.Dir, nil, stk, nil)
+		return loadImport(path, parent.Dir, parent, stk, nil, mode)
 	}
 
-	p := load(arg)
+	p := load(arg, mode)
 	if p.Error != nil && p.Error.hard {
 		errorf("%s", p.Error)
 		return
@@ -195,7 +199,7 @@ func download(arg string, parent *Package, stk *importStack, getTestDeps bool) {
 	// Only process each package once.
 	// (Unless we're fetching test dependencies for this package,
 	// in which case we want to process it again.)
-	if downloadCache[arg] && !getTestDeps {
+	if downloadCache[arg] && mode&getTestDeps == 0 {
 		return
 	}
 	downloadCache[arg] = true
@@ -255,7 +259,7 @@ func download(arg string, parent *Package, stk *importStack, getTestDeps bool) {
 		pkgs = pkgs[:0]
 		for _, arg := range args {
 			stk.push(arg)
-			p := load(arg)
+			p := load(arg, mode)
 			stk.pop()
 			if p.Error != nil {
 				errorf("%s", p.Error)
@@ -291,28 +295,26 @@ func download(arg string, parent *Package, stk *importStack, getTestDeps bool) {
 				continue
 			}
 			// Don't get test dependencies recursively.
-			download(path, p, stk, false)
+			// Imports is already vendor-expanded.
+			download(path, p, stk, 0)
 		}
-		if getTestDeps {
+		if mode&getTestDeps != 0 {
 			// Process test dependencies when -t is specified.
 			// (Don't get test dependencies for test dependencies.)
-			//
-			// We apply vendoredImportPath here.  It's not
-			// needed for Imports, because it was done
-			// while loading the package.
+			// We pass useVendor here because p.load does not
+			// vendor-expand TestImports and XTestImports.
+			// The call to loadImport inside download needs to do that.
 			for _, path := range p.TestImports {
 				if path == "C" {
 					continue
 				}
-				path, _ = vendoredImportPath(p, path)
-				download(path, p, stk, false)
+				download(path, p, stk, useVendor)
 			}
 			for _, path := range p.XTestImports {
 				if path == "C" {
 					continue
 				}
-				path, _ = vendoredImportPath(p, path)
-				download(path, p, stk, false)
+				download(path, p, stk, useVendor)
 			}
 		}
 
diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index 95b5eb347af2e..d04b52a6755b0 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -249,11 +249,29 @@ func makeImportValid(r rune) rune {
 	return r
 }
 
+// Mode flags for loadImport and download (in get.go).
+const (
+	// useVendor means that loadImport should do vendor expansion
+	// (provided the vendoring experiment is enabled).
+	// That is, useVendor means that the import path came from
+	// a source file and has not been vendor-expanded yet.
+	// Every import path should be loaded initially with useVendor,
+	// and then the expanded version (with the /vendor/ in it) gets
+	// recorded as the canonical import path. At that point, future loads
+	// of that package must not pass useVendor, because
+	// disallowVendor will reject direct use of paths containing /vendor/.
+	useVendor = 1 << iota
+
+	// getTestDeps is for download (part of "go get") and indicates
+	// that test dependencies should be fetched too.
+	getTestDeps
+)
+
 // loadImport scans the directory named by path, which must be an import path,
 // but possibly a local import path (an absolute file system path or one beginning
 // with ./ or ../).  A local relative path is interpreted relative to srcDir.
 // It returns a *Package describing the package found in that directory.
-func loadImport(path, srcDir string, parent *Package, stk *importStack, importPos []token.Position) *Package {
+func loadImport(path, srcDir string, parent *Package, stk *importStack, importPos []token.Position, mode int) *Package {
 	stk.push(path)
 	defer stk.pop()
 
@@ -268,7 +286,7 @@ func loadImport(path, srcDir string, parent *Package, stk *importStack, importPo
 	var vendorSearch []string
 	if isLocal {
 		importPath = dirToImportPath(filepath.Join(srcDir, path))
-	} else {
+	} else if mode&useVendor != 0 {
 		path, vendorSearch = vendoredImportPath(parent, path)
 		importPath = path
 	}
@@ -277,8 +295,10 @@ func loadImport(path, srcDir string, parent *Package, stk *importStack, importPo
 		if perr := disallowInternal(srcDir, p, stk); perr != p {
 			return perr
 		}
-		if perr := disallowVendor(srcDir, origPath, p, stk); perr != p {
-			return perr
+		if mode&useVendor != 0 {
+			if perr := disallowVendor(srcDir, origPath, p, stk); perr != p {
+				return perr
+			}
 		}
 		return reusePackage(p, stk)
 	}
@@ -334,8 +354,10 @@ func loadImport(path, srcDir string, parent *Package, stk *importStack, importPo
 	if perr := disallowInternal(srcDir, p, stk); perr != p {
 		return perr
 	}
-	if perr := disallowVendor(srcDir, origPath, p, stk); perr != p {
-		return perr
+	if mode&useVendor != 0 {
+		if perr := disallowVendor(srcDir, origPath, p, stk); perr != p {
+			return perr
+		}
 	}
 
 	return p
@@ -851,7 +873,7 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		if path == "C" {
 			continue
 		}
-		p1 := loadImport(path, p.Dir, p, stk, p.build.ImportPos[path])
+		p1 := loadImport(path, p.Dir, p, stk, p.build.ImportPos[path], useVendor)
 		if p1.Name == "main" {
 			p.Error = &PackageError{
 				ImportStack: stk.copy(),
@@ -1524,7 +1546,7 @@ func loadPackage(arg string, stk *importStack) *Package {
 		}
 	}
 
-	return loadImport(arg, cwd, nil, stk, nil)
+	return loadImport(arg, cwd, nil, stk, nil, 0)
 }
 
 // packages returns the packages named by the
diff --git a/src/cmd/go/test.go b/src/cmd/go/test.go
index 668665053dd3b..baeec9d3a9842 100644
--- a/src/cmd/go/test.go
+++ b/src/cmd/go/test.go
@@ -583,7 +583,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 	var stk importStack
 	stk.push(p.ImportPath + " (test)")
 	for i, path := range p.TestImports {
-		p1 := loadImport(path, p.Dir, p, &stk, p.build.TestImportPos[path])
+		p1 := loadImport(path, p.Dir, p, &stk, p.build.TestImportPos[path], useVendor)
 		if p1.Error != nil {
 			return nil, nil, nil, p1.Error
 		}
@@ -614,7 +614,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 			pxtestNeedsPtest = true
 			continue
 		}
-		p1 := loadImport(path, p.Dir, p, &stk, p.build.XTestImportPos[path])
+		p1 := loadImport(path, p.Dir, p, &stk, p.build.XTestImportPos[path], useVendor)
 		if p1.Error != nil {
 			return nil, nil, nil, p1.Error
 		}
@@ -749,7 +749,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		if dep == ptest.ImportPath {
 			pmain.imports = append(pmain.imports, ptest)
 		} else {
-			p1 := loadImport(dep, "", nil, &stk, nil)
+			p1 := loadImport(dep, "", nil, &stk, nil, 0)
 			if p1.Error != nil {
 				return nil, nil, nil, p1.Error
 			}
diff --git a/src/cmd/go/vendor_test.go b/src/cmd/go/vendor_test.go
index b99b4f1185658..ac32545b3b0f9 100644
--- a/src/cmd/go/vendor_test.go
+++ b/src/cmd/go/vendor_test.go
@@ -9,6 +9,7 @@ package main_test
 import (
 	"bytes"
 	"fmt"
+	"internal/testenv"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -173,3 +174,15 @@ func TestVendorGet(t *testing.T) {
 	tg.run("get")
 	tg.run("get", "-t")
 }
+
+func TestVendorGetUpdate(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+	tg.setenv("GO15VENDOREXPERIMENT", "1")
+	tg.run("get", "github.com/rsc/go-get-issue-11864")
+	tg.run("get", "-u", "github.com/rsc/go-get-issue-11864")
+}