These are user to user forums, so no-one here can tell you why you were charged for a second month.
However, I know that Magix will automatically charge you for each subsequent month if you do not cancel the subscription, normally at least 30 days before the next payment is due. So, if you do not very quickly notify Magix that you wish to cancel the subscription you will shortly be charged for another month. Details of how to cancel your contract will be in the email you received when you originally started the subscription.
Also, please be aware that the correct forum for questions about Vegas Pro is here.