@nansies I would advise approaching this in two ways:Hi! If you have already accepted a competing VS, is it best to let the second firm know and ask about switching schemes once (if!) you receive the second VS offer or before/during the AC?
1. Let them know if you have been offered another Vacation Scheme with conflicting dates, and you wanted to know if your AC place could be considered for a different scheme instead. This gives them the chance to move you over into another pool, and offer the current AC space to somebody else instead
2. If that doesn't work - I'd still attend the AC, and if an offer is made, ask both firms if either can move you to another scheme - and if so, great. If not, go with the one that you prefer the most.
It's not going to reflect badly on you, so long as you make sure you sound interested in both equally.