I actually bought that same bike recently to fix up as a commuter. My impression when the bike arrived was that the original crankset was the weakest component on the bike. The middle chainring was indeed already bent on the one I purchased (a manufacturing defect) and if, I remember correctly, there weren't even ramps and pins to to improve shifting. I would just get a different crankset if you are certain the problem is due to a bent chainring and not a simple cable adjustment. I certainly wouldn't bother replacing chainrings on such a basic crankset. I replaced mine with a Shimano FC-2303 which essentially is in the same parts group as the front derailleur so it ended up working fine as Andrew's post suggests. I did have to change the bottom bracket as well which I was happy to do anyway. I suspect the Vuelta crankset should work if you want to go that way. Just make sure you use the proper bottom bracket.

Mitch