So I made a new site and I use a backup of the forums folder for my other webserver and get an error, what do I have to do to fix it?
Probably you have to change something in your settings to make it recognize the new webserver.
I know mybb, xenforo and vbulletin require those edits, maybe vanilla works like that aswell
Welcome to the community.
It would help if you were more specific about what you did.
As a minimum, when you move the location of your Vanilla folder you need to delete any ini files in the cache folder.
The information (users/discussions etc.) for Vanilla is held in a mySQL database, so you need to make sure that the Vanilla installation has access to that.