Wasting my day at work coming up with solutions for ya.
I thought there was a Vbul code to alter the Control Panel to make certain options fixed via a Usergroup. Could have sworn it was on one of the other sites I'm on. I'll look into the other sites I'm on and let ya know if I see it.
Far as PM's. Personally, in my experience there have been two methods to this. One increase in size for payers, or have it turned off for freeloaders. But still allow the 'Email User' function.
Avatars - another Vbull site only allows avatars for paying members. You say the option for anitmated Avatars is turned off, but isn't there a sizing/file option you can split for Usergroups? I've seen it done on others, where only paying members could have the anitmated avatar.
Adspace - Paying vendors expect ad space to all your membership. Remember that's the lure for bringing them to spend money on your site in the first place. "Yo, Yeti Inc, we have 1,000 paying members. Would you like some ad space?" If you turn my ad off for those 1,000 members what am I paying for?
Bearing that in mind, this is where you can leverage your 'Paragon Tier'. Allow it the full no ad experience. Than you can go we have 3,000 Forum Members with 1,000 of them subscribers with about 100 in our Paragon level. The vendor will see a possible 3,000 customers out of it.
Offering Vendors their own forum to answer questions on their wares or fulfill requests can be helpful as well.
On one of my Jeep Forums, Vendors pay $100/yr. They have their own section of forums where they can conduct business like a store front.
I.e. Billy Bob asks Yeti Inc if they can make him a bumper to these specs. They answer Billy Bob and generally use the PM's to negoiate the custom bumper price. Or they answer in thread saying yes we have 3 in stock for this price, yada yada yada.
Dice Roller - very nice to have and could be worth it for some subscribers. But than again, Invisible Castle offers the same thing for free with code to put on the message board to show your rolls.