No, you don't have to turn it off. You might have to make sure that the permissions are ok -- or -- run your BlitzMax environment with administrator privileges (as you have to do it with Visual Studio, by the way).
Easier just to turn the ruddy thing off, than try and figure out all that other permissions stuff.
The one exception is one of my game's save files went to some unknown place, I turned off UAC and the files appeared where they belonged, but settings for another file got lost, I turned it back on and both programs are working correctly.
I don't think so. It just "looks" like it's now working correctly. The problem is still there, which you're potential customers or other users may soon find out - especially if they use multiple accounts on the same PC.
I'd rather keep my day job and sell 100 copies of something I enjoyed working on than sell 10,000 copies of something that was a chore to develop, while simultaneously helping to support the very platform that made my fun project into a chore.
There's very little extra work to do to make your games work under Vista, as all the "work" has now been done by various users of this forum (particularly for BlitzMax developers). All Vista has really done is forced us to do things the "proper" way from now on where save files and permissions are concerned, whereas everyone, including the big boys, were just lazy and slapped all save files within the game's directory (to name just one example).