This is (tentatively) working. It uses regular expressions to try and parse variables out of the current line.
Closing a tree node that ends the watch window buffer no longer causes a memory error.
Hitting <enter> on a tree node toggles that node, so whereas before it would only open, it can now close.
Run and set breakpoint keys are available when Vim starts, but all other key maps are set when the debugger is started, and unmapped when the GUI is closed. Also, a leader key can be set with g:vdebug_leader_key
Option set up is done at Runner.__init__() as well as Runner.open(). This fixes a bug with trying to set breakpoints before running the debugger.