Short of having both units nothing is for certain.
However as the HC7 will output HDV on its firewire port I have a hard time working out a way it couldn't work. What would make it work better would be whether or not the HC7 has that syncro record function or not. If you have the HC7 the manual will tell you. What that feature does is when you press record on the camera it send a command down the firewire port to any compatible device to make it go into/out of record.
One other check you could try if you haven't done this already.
Run a firewire cable from the camera into your Vegas system.
With the camera in Camera mode if you can capture HDV with Vegas then the MRC1K must be able to do the same thing. The record start/stop functionality is really the icing on the cake. iI the full functionality of that feature is enabled then you also get status feedback from the recorder buy the only camera I now for cetain having used it is the V1 with the Sony HDD recorder (DRU60?).
I had one of these units on loan (came with the Sony270 cam)
I too was curios to see if it would work with another camera
I tried it on a JVC 5000 via fire wire and it worked OK turned of and on via FW no problem,
I would go for the big battery so you can leave the device on all day and not worry about turning (power) it on and off