Hi Bill,
The options are very limited - DosBox may be the best(and perhaps, the
only).
Vista uses a new video driver model (so-called "WDDM") which allows the
fancy composition displays in Vista. The Windows developers explicitly
dropped support for the full-screen VGA Mode, in the WDDM code. There
were considerable technical problems in making that full-screen mode
work in the new WDDM regime, and it was deemed "not worth it" for the
small number of user scenarios where character-mode apps needed to run
full-screen on a Vista machine. This issue was reported several times
during the Vista Beta programme.
Dedicated devices like POS cash registers generally run Windows
Embedded, not Vista; so they didn't count.
Regards
Andrew