I believe that the vulcan 500 actually has more power than a H-D 883, it is actually a fast little bike. After some time on the 250 pretty much any cruiser between 650cc and 900cc would be a good upgrade from the 250. V-Star 650, V-Star Bolt, V-Star 950, Vulcan 750, 800, 900, Suzuki S50, C50, M50, H-D 883 would all be good upgrades from a 250, some of these even being good starter bikes on their own (V-Star 650, H-D 883, V-Star Bolt, and maybe the Vulcan 900). Most of these are in the 40-50hp range and most of the metric cruisers in this size range made within the last decade or so are very well balanced bikes and easy to handle.
If you want a point of comparison, I started on a Honda CX500 which was a V-twin standard with about 50hp, at no point did I ever feel overwhelmed by the size or power of this bike (it was fairly heavy but not huge and the engine was great for a 500cc), after a few months on this I test rode a H-D 1200 and felt comfortable with its torque (huge torque but quite controllable, but would probably scare a brand new rider). After 1/2 a season I ended up selling my CX500 and upgraded to a Suzuki Bandit 600 which has about 85hp and made even that H-D I test rode look slow. I found the jump from the 500 to the 600 to be about perfect for me.