That's right, you can always disable a plugin and any changes will be undone on the next page load.
On the other hand you can also always "Revert" a template back to the VB stock version thereby undoing any changes made to it as well.
And as a last ditch scenario if somehow you made a plugin preventing you from logging into the admin cp- you can disable all plugins via config.php file as described here:
https://www.vbulletin.com/forum/show...ystem-globally