As an association, you should ask, and even possibly demand with urgency, that these other websites participate by making this forum part of their online connection.
Perhaps, with every delivered item, a printed notice to... "Support the continued use of your electronic vaporizers. News, Info, Suppliers, Help, and more. Please visit http://www.e-cigarette-forum.com"
I believe this forum does a great job of being neutral to the point of including multiple sellers, and without showing any favoritism. There is also a direct separation of the forums provider, from the actual forum. (I though this was unusual, but I like what the arrangement portrays. I had to dig, before I realized who hosts this forum.)
In light of staying in business, it is important that they see and understand the importance of this single forum. (I know of no other e-cig forum that is frequented or as informative as this one is.)
Perhaps you can persuade them, by pointing out that GOOGLE has this forum ranked as the number-one e-cig forum, and every e-cig key-word leads back to this forum, within the first ten links in almost every GOOGLE search. (I have not tried any other search engines.)
Perhaps you could redesign the front-page of the forum, to be more inviting to every single seller. So as to stay completely neutral, and offer leveled exposure based on paid support.
I would love to assist with development of an auto-mailer, which would create a semi-generic "Friendly", letter to e-mail or print and snail-mail to any selected representative. (I don't have the ability to write the actual content of the letter, but once written, I can segregate it, and add enough random uniqueness to make every letter seem genuinely unique. Since many of us can agree with certain things, but placing them into words can be a stopping point. That is why a majority do not take the time to write. Writers block.)
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<A HREF="Mailto:test@mail.com?Subject=Title&Body=Content">TEST LINK</A>