So, I've been trying to play the "Path of Evil" module from the Vault, but every time I try to load it, I get the error message "You need to install Expansion Pack 1: Mask of the Betrayer." I haven't yet tried to reinstall, but I'm wondering if that would even help?

Using the Nwn2 Complete from GoG, fully updated. Also have Kaedrin's and the Tome of Battle hacks installed.

Try to clean up your override. Tome of battle might have some override edits that trick game into thinking that you has just basic game. Kaedrin is not a culpit here - I played path of evil with it