In a true college-level calc-based E&M class, which AP Physics C is not as much as the CB would like to pretend that it is, MVC is often a prereq and is used extensively. However, few high schools offer MVC so you will rarely, if ever, need it for the AP exam. Some problems which could easily be solved using MVC, can generally be reduced down to solve with lower-level math. Regardless, the E&M exam has a generous curve with a 60% needed for a 5. Self-studying E&M will be difficult as it is; don’t spend a boatload of time on concepts that will be very infrequently needed.