i suggest a good server with alot of ram, quick HDD and a good processor. If you talk about a few thousand people, webspace wont/cant handle it.
You will probably need a server which is only for the Database.

Would you care to elaborate how you come to this suggestion?

Easy... 5000 members and lets say you got 1500 online. If he is hosting other things, like a gallery or website which uses mysql it will slow down everything. vb is using the Database for everything, that means always being connected with the Database. 1500 users will cause alot of traffic and processing power. Having a server just for the DB, will speed up the ´thing alot for a simple reason:

Most hosters = if you have 2 servers at their network, both servers will be connected with a 1gbps connection. You will still have the 100Mbit connection at your website server, but the processing between script and database will be much quicker. Your website server will just give you the result of the MySQL server processing, and almost nothing will be slowed down. If you host everyting on one server, you will have massive lag.
But: If you have a forum with lets say 620.000 members and arround 50.000 members online you can use it on one server, but you will need at least a QuadCore (better use 2x Quad), 8 or 16GB Ram and SCSI hdds with 15.000 UPM. And 100MBit wont handle it. a 1Gbps connection is a must be for this.

But i just have read the thread again, and one server is enough for that. I thought he ment 5000 users online at same time.

The number of users online is not a real issue here. It is the amount of forums that leads to problems.