Converting to an IPS community
No! When you convert from vBulletin, we include free scripts which automatically translate your current URLs into IP.Board 3's Friendly URLs. Appropriate headers are sent, so search engines don't penalize you for the change, either.
Unfortunately yes - skins and modifications designed for other software will not work with IPS software. But similar modifications and skins are probably available via our resource site, contributed by other users.
This depends on the size of your current database and the number of entries, so it is difficult to give a definitive answer. Small communities may convert in a few minutes, while large communities may require several hours. Once the conversion is running, however, it can be left to complete without intervention.
Yes, users will be able to log in normally once the conversion is complete.