That's when I realized that I needed inner and outer envelopes. Vistaprint only came with one envelope per invitation (as any normal person, myself included, would expect).
If you decide to go with Vistaprint for invitations you have two options:
1) Use the Vistaprint envelopes as inner envelopes (since they just fit the invitation) and then buy outer envelopes from somewhere else.
2) Don't use Vistaprint envelopes at all and buy a matching set of inner and outer envelopes.
We have opted for Choice #1. Since our wedding colors (and therefore our invitations) are dark purple and white, we will be using the white Vistaprint envelopes on the inside and will get dark purple envelopes for outer envelopes.