It comes down to whether you are selling on your own or as a group.
Choose Single Vendor if you offer your own services straight to couples, like a photographer running their own business, a makeup artist working solo, or a caterer on their own. Choose Vendor Team if you want to join up with other vendors already on The Aisle to sell couples one complete package. For example, a photographer, a decorator, and a caterer offering their services together as a single deal, run by one team leader.
You are not locked into one or the other. Many vendors keep a Single Vendor listing for their own work and also join a team or two. Being in a team does not stop couples from hiring you on your own. If you are just starting out, a Single Vendor listing is the natural first step, and you can create one for free.