Does Lady Vix's Level Up force her to lose her current face-up Item?
If a player has two Item cards face-up in their scoring area and no ability allowing more than one, the player chooses which one to turn face-down.

See the image of the Tools Of Hero Kind expansion rules here, which says in parenthetical


  You choose which item to keep face-up


when speaking of a player with more than one face-up Item card.