"how do i know how much vintage postage i need?"
Great question that is best left to the pros at the US Post Office. You can take an assembled invitation to the post office to have it weighed, and you can also use the USPS Postage Calculator to get a good estimate.
Here are a few general thoughts and estimates to give you an idea of what you might need:
If you are mailing a standard sized rectangular envelope that weighs less than 1 oz., you will need 1 "forever" stamp, or 50¢ worth of postage. Keep in mind that postage rates do fluctuate up and down over time, so double and triple check before you apply your vintage postage!
If you are mailing a square envelope that weighs less than 1 oz., or a rectangular envelope that weighs between 1-2 oz. (most wedding invitations) domestically, you will need 71¢ worth of postage.