Cam chain tensioners are key to to maintaining a good 1500.
The reason is mysterious but it seems one of the first things to give any trouble on Vulcan 1500/1600 engines is the cam chain tensioner. That's the spring loaded gizmo that keeps an idler pressed against the chain so it doesn't whip or come off the sprocket or even make nasty grinding noises as it stretches through normal use. At some point though the tensioner reaches the end of its travel and then, when the chain stretches just a little more it starts rattling, usually against the spark plug tunnels (tubes) causing damage. The chain can eventually wear a hole in those tunnels which then fill (slowly) with oil and suddenly the rider notices oil seeping down the side of one cylinder. A little more wear and really bad things can happen. Most folks don't know this and address it, so that makes it a common problem on bikes with 30,000+ mileage.
The lower end is pretty solid, as long as it's maintained well.....and that's the problem there. Many folks don't change the oil and filter regularly, and do it correctly. There is a bolt on the lower left side of the engine that is the best place to drain oil from (not the drain plug). It's actually lower then the drain plug when the bike is on the side stand, and it contains the screen filter for the motor. When I got my ride at 18,000 miles, I pulled this plug and found some stuff I wouldn't want floating around in my system. Check this out: http://www.gadgetjq.com/vulcan_1500_oil-sys.htm