Click the mail tab at the top of the page.

Re: If you have VIP membership, you can send a PM by

clicking on the envelope next to the username of a poster when you've opened the post,

clicking on the links in the review profile pages that say "send private mail to reviewer" or "send private mail to provider" or

through the "mail" menu at the top of the page.  That menu is pretty self explanatory, but you need to be exact when you type in the username.

Once you send the PM, you will not know if the person is another VIP until that person replies (except for providers with a "see my ter reviews" link next to their names in a post, as they are given vip with they register).  Since you don't really know, some posters will reply to the post with a "You have a PM" in the subject to let the person know - it gives the recipient a chance to let you know that he/she isn't a vip member and provide an email address for the message.

Good luck