You would probably need several

One to host the database, and the other for the actual vBulletin software. wht has 2 quad xeons running their forums (one setup for mysql and the other for simply vB itself).
You'd need someone very talented and knowledgable to tweak the servers up beforehand as well

I think with 3000 users online that might struggle a bit as well (may have to split the database in 1/2 onto two servers). vBulletin can handle it, it's just you need enough grunt behind it.