Open the vendor’s profile page. On it you will find their contact options, usually a button to message them directly through The Aisle, a phone number, and sometimes a website or social media links.
The simplest way is to message them through the site. For that you will need a free account, so the vendor can reply to you and all your messages stay in one place.
When you write, give them the basics up front: your wedding date, where it will be, roughly how many guests you expect, and what you need from them. The more they know, the faster they can tell you whether they are free and what it is likely to cost.