The entrancing thing about the world of the apartment association is they symbiotic nature of it all. The association doesn't really exist without the support of the vendors, and the value of the association isn't there without the apartment members.
Up here in Seattle, we have a really young apartment association (7 years old this year I think!), but we're growing by leaps and bounds because people are starting to really see the value of being involved on both sides of the spectrum. Some of this was brought about by nothing more than talking one on one to each member and being willing to extend a helping hand. Other times, it comes from vendor to vendor discussions about the best way to create and keep business with our clients. I sit on WMFHA's supplier council and LOVE it. Of course, our association has a very personal approach and is well run, so we have that on our side
Have you considered putting together a reverse trade show to help your vendors see the value? 20 minutes in front of all the different PM companies in the area is DAMN valuable time real estate to most people.
And, as both Brent and Daisy said, it comes down to taking the risk and jumping in to the participation pool. They will never see the value until they actually just do it. The next time they tell you they can't afford the time, ask them how they can afford not to commit the time? Time to network and work with the very people they're trying to get in front of? Time to build the personal side of the relationship, which inevitably leads to the business side? I think they can afford that. It just comes down to perspective.
Sometimes all it takes is one domino to get the rest to fall down. Find the key players, make sure that they meet with success, and you'll get the rest.