Defines defined in and included from header files are not defined when analyzing source files.
File structure:
.
├── src
│ ├── define.vh
│ └── top.v
└── vide.toml
vide.toml
sources = ["src"]
include_dirs = ["src"]
define.vh
`ifndef __DEFINE_VH__
`define __DEFINE_VH__
`define SOMETHING
`endif // __DEFINE_VH__
top.v
`include "define.vh"
module top (
input wire clk_i,
input wire d_i,
output wire q_o
);
reg q = 0;
`ifdef SOMETHING
always @(posedge clk_i) q <= ~d_i;
`else
always @(posedge clk_i) q <= d_i;
`endif
assign q_o = q;
endmodule

Defines defined in and included from header files are not defined when analyzing source files.
File structure:
vide.toml
define.vh
top.v