I don’t think there is a “right” folder - I think its understood that really you can create a folder anywhere and put your vst’s in it and that’s fine. Not the most organised, but so long as you point Renoise in the right place…that said:
Which operating system are you using?
If your using Windows 7, take a quick look at This Post. By putting your VST folder on your data partition, or in a public or home folder, you can avoid getting the errors from synthedit plugins (Basslane, for example).
On the other hand, I don’t recall XP being so fickle by default so if you really wanted to bury them in your Program Files or System Folders I don’t think it would complain 