DaveC E100, the DVD recorders you mentioned are all standard machines designed primarily to record TV. You are of course correct that almost none of these will recognize a FireWire signal unless it comes from a camcorder. (There were a couple of early Pioneers, models 310, 510 and 520, that could often be tricked into accepting other FW sources).
The Sony VRD-MC6 is a tiny little recorder with no tuner, not much bigger than an old CD DiscMan player. It is dedicated specifically to making digitally transferred DVD backup copies of videos stored on modern Sony HiDef and SD camcorders, or JPEGs from compatible camera cards. The biggest difference between the VRD-MC6 and "regular" DVD recorders is that the Sony is capable of receiving a digital HiDef camcorder feed and preserving the HiDef as an AVCHD-format DVD playable on any BluRay hardware. AVCHD will not play on DVD hardware at all, so if desired the VRD-MC6 can optionally convert the HiDef to a standard-def DVD.
All of this sounds peachy and convenient, but in practice these little Sony DVD accessories are known to be very buggy and picayune. They generally work OK with Sony cameras and Sony memory cards, but with anything else you're SOL. The VRD-MC6 balks at many non-Sony memory cards, will not accept an external reader on its USB port, and rarely recognizes a non-Sony camcorder via USB or FW. You can connect non-Sony gear via traditional analog inputs, but this kinda defeats the purpose and offers no advantage over an ordinary household DVD/VHS combo recorder. The VRD-MC6 also has the most hypersensitive copyright protection imaginable: it will flatly refuse to record more than 50% of normal analog camcorder videos, incorrectly identifying them as copy-protected.
I would not recommend a VRD-MC6 unless you specifically own compatible Sony devices. Now that I think more about it, ThumbtackJack, it almost certainly WILL NOT work via FireWire with your JVC HM-DH400U. There's a small chance it might, since your JVC would likely identify itself over FW as a "digital tape," but be sure you buy from Amazon or WalMart or someplace else with a no-questions-asked refund policy it it fails to work. Sony never marketed DVHS itself, so VRD-MC6 may not recognize the protocol in any case. You may want to investigate PC video add-ons like Hauppage or Black Magic Intensity instead of the VRD-MC6, they're not as convenient to use but offer more compatibility and video format options.