You don't need to replace both unless needed and you can mix brands. I had a Metzler front and a Michilin rear on my FJR and it handled fine.
The only thing you don't want to do is mix a radial tire with a non radial tire. The Vulcan is not designed for radial tires.
Edited to fix typo. Thank you spell correct for being such an pain sometimes.
Hey Km, I don't know where you stand on taking a cruiser, specifically a VN750 to the "darkside" by installing a car tire on the rear wheel, but I have a radial car tire waiting at the shop to install on my scoot. I have a fairly new front tire on that is not a radial.
craznurs put a radial rear tire on her vn700 and ran it hard through the twisties for 15 or 20k miles before selling it when she moved up to bigger bike. Her new bike was a 1600 V-Star which already had a car tire installed on it. She said she would never go back to a mc tire on the rear of a cruiser. A similar sentiment has been expressed by almost every rider I have read on darkside forums who has taken a cruiser or bagger to the darkside.
A few other guys here have tried it too, and all have used radial car tires for the rear as far as I can remember.
I seem to recall that you mentioned several years ago that some sport-touring bikes, such as your FJR(?) where also using car tires on the rear wheel.
Is the FJR designed to use radial tires?
EDIT: OK, just checked specs for the 3 motorcycles mentioned. The Valkyrie and FJR do come with radial tires from the factory.
The Yamaha V-Star does not.
For anyone not familiar with this idea, Daniel Meyer gives the most comprehensive write-up of why he did it to his 2001 F6 Valkyrie 1500, and his experience with it. The Valkyrie is refered to by some as a "power cruiser". Daniel purchased the bike new. Well in excess of 100k miles on the darkside at the last update on Dec. 5, 2007. Probably over 200k by now.