If you are running phpbb all you would have to do is download the files and upload them here. Do a full database backup.
You should only have to change one "config.php" to the new database information.
You can run vb, keep in mind that it is a little more resource intensive and if you're board has a lot of traffic and is hogging resources you might be asked to cut down on CPU usage or move to a dedicated server.
Well nothing is unlimited! I have about 35 and it isn't or hasn't been a problem so you should be fine.
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