I keep getting the message "An error occurred during payment. Please use a different payment method or contact customer support." All my info is correct, so what's causing this? I'm literally trying to give them money and they're like "NO!".
Please understand that this is a user forum and not a support website. You will need to contact Vegas Pro sales directly via one of the avenues shown here:
The stock advice you'd get using https://support2.magix.com/customer/en/request and describing your problem as "credit card" is as follows:
"Why am I having problems ordering with a credit card?
Please be aware that our online Webstore is based in Germany and requires an international payment. If there are problems with ordering, then we recommend to check with your card provider and make sure that they allow international payments.
If that does not help, then we recommend to try a different card or payment method, like Paypal."
If you want to confirm the cause, please ask MAGIX (as @Dexcon suggests) e.g. using the request link.