Yeah vbulletin would probably be even worse (as an aside I'd suggest Xenforo if someone is looking for commercial forum software, seems good, don't really like what vBulletin has been doing lately).
Without some further debugging it'd be hard to know what's actually the cause and know what kind of tweaking could maybe alleviate the issue.
You can try to move to a shared host that's more forum friendly, phpbb has some they recommend:
https://www.phpbb.com/hosting/
There's also hosted forums, where someone else does all the work of hosting, presumably they put advertising in to pay for it, might be worth checking. I'm a big fan of having someone else do things
http://www.prophpbb.com/
http://www.phpbbserver.com/
I have no experience with them though so I don't know which one would be good (i.e. one that wouldn't spam your users or keeps their ads to a reasonable level).