"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 & measured, and you can also use the USPS Postage Calculator to get a good estimate.
Here are some general guidelines to keep in mind:
Envelope Size: For your envelopes to qualify as a “Letter”, the envelope must meet the following guidelines:
The envelope must be rectangular, and must be at least 3-1/2” high, and 5” wide (an envelope that is 3” x 6” would not qualify because the shortest side is shorter than 3-1/2”).
The envelope also cannot be larger than 6-1/8” x 11-1/2’ (an envelope that is 6-1/2” x 9” would not qualify because the “shortest” edge is longer than 6-1/8”).
The nuances here can be confusing, so please take your envelope to the post office to confirm (more detail on USPS letter sizing here).
Non-Machinable Characteristics: If you are adding a wax seal, a button, external ribbon, or if you are writing the address sideways, or if you have an invitation that is “too rigid” (along with a handful of other nuances), you will likely also need to pay a “non-machinable surcharge” as your envelope will require hand processing. As this time, the surcharge is $0.21. Again, the recommendation is to take your fully asembled invitation to the post office to confirm. (more details on USPS non-machinable characteristics here)
If you are mailing a standard sized letter 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! There is a postage rate change scheduled for January 2019.
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.