Just a random question, but do vendors resupply in Morrowind? Specifically, magical scrolls (Like the Unhinging scrolls)?

I'm asking because my low-combat build current character is finding a lot of utility out of various scrolls, leading me to potentially make a Thief/Scroll Mage build.

It depends on the vendor and the item. Each vendor's inventory is partially one-offs that are not restocked, like the Dreugh items at Meldor's, and things that do restock, like Meldor's Netch Leather items. Just because something does not restock at one vendor does not mean it never restocks at any vendor. I'm pretty sure that Mages Guild vendors tend to restock their scrolls, though.