Alan, this is an interesting idea. I wonder how I would replicate your setup in order to try to replicate your problem? I am not sure why this is happening, UNLESS ... does Xvesa make some assumptions about the location of tables or information it uses to start up? What does Xvesa do? The only VESA stuff I know about (in plan 9) might be dependent on a fixed BIOS location for some things.
ron