[bobetter]

Most of the Class B (which I assume you want to use versus Class A) gear out there is designed around board(s) made by SRT (Software Radio Technology). As a result, the performance and options are all pretty similar from manufacturer to manufacturer. There are a couple of "Mavericks" but I've not seen a lot of coverage on them which probably means they are on par with the SRT stuff. There is one unique box out there called "WatchMate" which is designed for the ocean cruiser and will perform very well as a standalone unit as well as interfacing with a full blown nav system. If you Google "AIS Class B", you'll get more results than you can imagine. Panbo's has done a fantastic job of reporting on an almost daily basis as new gear comes on the market, just click on the AIS topic.
In terms of piggybacking your AIS rig on top of your existing VHF install-there are a couple of draw backs to consider. If you are contemplating a receive only system, there are no real issues other than the obvious facts that the VHF TX will take precedence over the AIS receive and if your splitter fails, you run the risk of frying your AIS by transmitting on the VHF. If you want to also install a TX/RX system, I'd recommend a discrete antenna system. Sharing the VHF antenna with both the AIS and VHF TX could result in missed broadcast slots from you.