First Of All Never Use Vista For Gaming
As usual you have no idea what your talking about. I doubt you can name a single legitimate reason to "never use vista for gaming".
Try Changing Your Graphics Settings (Altough It Wont Work)
This doesn't even make any sense. Second, if it won't work, why mention it? It's like saying, "oh, your PC won't power up? Oh ok, try putting a plug in your bathtub, even though it won't work.
What makes it even better is it might work to change the graphics detail in the game to other values, as with resolution. Of course I know the specific settings rather then just blithly saying "change ur graphics settins"
Try Running Same Game On Other Operating System (XP)
This might not be possible.
Anyway, with regards to AOE2, I can play it just fine, with some visual artifacts from transparent layers on the ocean and grass and so forth. But I've never had units dissapear. I assume your loading from a saved game, correct?