The site mail link is in the top, right hand corner, beside "welcome" and your username. It is the little white envelope.
Click on the envelope and then the "compose message" link.
In the "To" box, you will need to enter the vendor's username, type what it is regarding in the "subject" box and then type your message in the "message" box and then click the "send message" box.
Keep an eye on your site mail as that is how the vendor will respond. You will see "New Mail" in white next to the site mail link when you have a new message.
You can contact other members this way as well.