Well every backrest is different, but, I have the Mustang backrest on my mustang seat. It's a whole new world of comfort on a long trip, it supports your back, it also give you a platform to lean against when shifting weight or stretching on a long non-stop ride.
Wife has never complained about it. She doesn't have a problem hanging on to me if she chooses to. It doesn't seem to get in her way. It's there, but, it doesn't bother her. One thing I've found is that Kawasaki cruisers (and the 900 I assume you ride based on your avatar is no exception) give a tremendous amount of passenger room, compared to other brands. Even big touring bikes like the Harley Davidson Electra Glide don't give as much passenger 'space' as the 900. So the backrest is less of an issue on a Vulcan than it might be on other rides.
If it's a concern, though, I'd suggest going with a Mustang or Corbin seat with backrest, as opposed to a seperate backrest for your stock seat (such as a utopia). The reason is, they mount IN the seat, sit a little lower, and take up less space. The misses will likely have more room with a 'built in' backrest.
"8 But God demonstrates his own love for us in this: While we were still sinners, Christ died for us."
Romans 5:8 (NIV)
2006 Vulcan 900 Classic LT "Couch-a-Saki/Mini-Bagger"
2011 Honda Shadow Aero 750 (Wife's)