The cost of one stamps is 45 cents and your wedding invitation will probably cost at least 60 cents with so many inserts adding to the weight. To place vintage stamps -- assuming their denominations are all between 1-30 cents -- you will need a huge envelope to get to the total you need to mail one invite. And remember that an oversized envelope will also add to the price of mailing.
I think a better option would be to design a vintage-looking stamp at zazzle.com. This way you get the vintage look but don’t have to worry about buying dozens more stamps to meet the rate you need or getting an extra large envelope.
Another option is to use vintage stamps on invitations for events with a limited number of guests -- bridal shower, bachelorette party, rehearsal dinner. This way the rate to mail is lower and you won’t have the extra cost of 100+ envelopes to find stamps for.
If you're looking for vintage stamps, Champion Stamps is a great resource.