I've noticed that there are several items that have more than one function. (ie Dawn Mahar Tome is a weapon(and filtered as a weapon) as well as a readable book) Would there be a way to filter items in shops/vault by function?

This option is available in shops/galleries, it's a little wonky but there are both Item Type and Item Use dropdowns on the Edit Items page. At least one should work for the type of items you're trying to find (personally, I find Item Use to be generally more reliable but I mainly use it to find minions to put on my zapper pets. I also used to use it to filter through wearables while I was working on the gallery achievements)

I feel stupid now, I didn't even notice that xD Thanks for pointing it out.

No worries c: Neither option seems to work 100% of the time so I'd love for it to be adjusted if possible. I don't know how often people use it though

Like said, this is already a thing in shops, but I would love to see it added to vaults as well.