My forums (admittedly, IPB, not vBulletin) has 200,000 posts ish and about 250 members, and its (gzipped) mysql file is about 70mb. TSR just went passed its 9million post mark. It also had a load more, even proportionally to posts, members than mine, but I dunno how much of the file they take up so let's ignore them. Obviously size of post etc matters, but we have a fair few beats on my forums too, so hopefully it's roughly proportional.
(9,000,000 / 200,000) * 70 = 3150
So I would guess around the 3 gig mark - maybe with the extra users 3.2gb (given that 3gB actually = 3072mB) when gzipped.
Of course, I could be entirely wrong...