[Outbackmark]

This can be quite complex but you can do it that way. There are probably a few different ways to go about it but here is how I do it.
You can have them running on the same DB but have 2 different installations, unless you are planning on doing an upgrade to a different version.
If you are updating the VB version, copy the DB, upload the new files to a new directory and then run the upgrade.php NOT install.php. This will convert the DB and make the necessary table changes resulting in a fresh install. Before running the script you will need to empty the style, product and plugin tables on the copied DB and edit the datastore table to remove references to the old plugins, though if the datastore table is not edited it will not cause any major problems apart from a few entries in the ACP Options. You will also need to remove any tables that have been added by products.
Once it's working you can simply change the folder names and upload the new CMS files to your public_html folder. (Don't forget to change the settings(URL etc) in Options when you change over)
It's probably a good idea to close the forum while doing this so you don't end up with new posts, registrations and PMs to deal with when you swap over.
The whole thing can be done in a few hours and if it does go wrong, just reopen your forum, recopy the DB and try again tomorrow LOL!!!
The easiest way I find to copy the DB is to use the VB (or one of the addons) to create an on server copy (backup) then use a script called Bigdump to insert it into the new DB, much quicker than PHPMyAdmin as it will only copy one table at a time, but it could be done that way too.