You should be able to pull this off, but as far as I know there is no Vanilla to bbPress converter.
However, the phpBB to bbPress converter works well. Since phpBB is quite popular, you should be able to convert from Vanilla to phpBB to bbPress.
That would be painful because you would have to create topic slugs manually for anything coming in from phpbb/vanilla
So has anyone found a way to convert painlessly from Vanilla to BBPress?
There both the same thing why would you even care to convert from one to the other