If bglmanx.dll works in the default Vista mode under Vista, which it's the expected behaviour, I'd say what needs to be fixed is the Pmdg one, which is forcing you to use XP mode under Vista to work.
No idea why the Pmdg wouldn't work under Vista mode, nor why bglmanx.dll wouldn't work under XP mode. You can try with the usual suggestions of turning off UAC and/or DEP, and see if it works that way.