The Vulcan S was mine before I got the RS and passed it on to my son...I can't say a bad word about it, I love that bike! It's great fun to ride, handles well and has plenty of get-up-and-go to be a blast to ride. What I tell people, especially new riders, is that it has plenty of power to be fun to ride but not so much that you have to worry about it running away from you. It's a great choice in that price range!
As for keeping up, it does just fine. Of course when I crank the throttle on the RS it takes a bit for my son to catch up on the S but that's to be expected as these bikes aren't in the same weight class. But again, the S is a blast to ride and the difference is a non-issue when we're out riding.