If I return one piece of a Fabletics outfit or bundle I bought with a VIP Credit, what do I get back?

You get store credit, not a partial VIP Credit. Fabletics can't split a VIP Credit, so if you return some pieces of an outfit or bundle, you get store credit based on the returned items' share of the total VIP price. If you return every piece, Fabletics refunds your full VIP Credit.