I also bought my wife a 1991 750 Nighthawk....the setup fits her, being that she is 5'4 and 108lbs....thank god she doesn't realize yet that her bike would rip mine to shreds if she chose to in a race. She's new to riding and the price tag on it fit $1,800 and it looks new, but has 22k miles.
Sounds like you and your wife have gotten yourselves a pair of nice bikes!
So far my wife has not been interested in riding two up, let alone riding her own bike!
My last bike was a Nighthawk 750. It was a 1993, uncrated and assembled in 1994, and sold to me in 1995 (with only 3000 miles on it) when the owner ran into financial difficulty. I can't remember how much I paid for it ($3000?), but the price seemed reasonable and the bike was in mint condition. I know I loved riding it. Unfortunately, I sold it after only a couple of years (and a few thousand miles) to get $$$ to buy musical gear. Big mistake. I've finally learned... NEVER sell a good bike, and NEVER sell a good guitar. Even if you're not riding/playing for a season. If necessary, take out a loan!
Anyway, when I bought my 2006 VN 750 last year, I was looking for something like the Nighthawk, and the Vulcan was the closest thing I could find among all the crotch rockets, off-road bikes, and massive cruisers. At first I was surprised by how loud the V-twin was compared to the inline four cylinder Nighthawk. But I've grown used to it and really love the bike. I've put more miles on it already than my last three bikes combined!
I'm intrigued by your comment that the Nighthawk would rip the Vulcan to shreds in a race... I was so long between bikes that my memory doesn't help me compare/contrast the two. (The cylinder count is one big difference!)