Hi!
Congratulations on the AC invite!!
I faced a similar situation last cycle when I was invited back for a final TC interview after my first summer VS, which unfortunately clashed with my second VS (+ there was no flexibility with dates). At the time, I reached out to a mentor who suggested asking the VS firm whether it might be possible to step away briefly to attend the interview. I was quite hesitant to ask, but the firm was actually very understanding and allowed me to leave for a few hours.
As Jaysen mentioned, I appreciate that not all firms will be as flexible - an AC is a bigger commitment than a short interview, so the situation is a bit different and the VS firm might not be comfortable with letting you miss a whole day of the scheme. That said, I don't think there is any harm in asking the VS firm's graduate recruitment team whether there is any possibility of accommodating the clash. Even if it is unlikely, at least you will know either way, and a polite and professional request won't count against you.
If they aren't able to offer any flexibility, you might then need to make a judgment call based on which opportunity you'd prioritise - for example, where you feel you have the strongest chance of securing a TC, or which firm aligns more closely with your interests and career goals. It isn't an easy decision, but at least you'll know that you explored all options before choosing!