If I ever start my next project, Im going with another Vbulletin. I thought about a free forum at first, and was going to use IPB 2, but it then became paid. Since I know VB well, Im going to bite the bullet and buy another license. If it does not work out, I can always sell it.
1) You can use your own domain. 2) Why would you need to download it? It's hosted for you and if you want to convert/transfer your forum, it is provided. 3) Okay, I'll give yoy that one, you can not modify the board software. But its free, it runs really fast, and if you really want to modify it, you can get javascript to do it.
How? according to the faq: What will my URL look like?... The URL of you board will look similar to s6.invisionfree.com/yourboard/ For backups... Ive never seen a javascript hack. Another limitation is the 16 meg attachment limit and you cant use Adsense to generate revenue.