This is something you could fix yourself using a pair of sheet metal seaming pliers.
These pliers have wide jaws and are basically a hand-held sheet metal brake. Using them should allow you to repair the damage. To do the repair, start with the most damaged part and slowly bend the metal back into shape.
You could then find some touch up paint that matches your siding as close as possible and paint over the spots where the paint chipped off.
I wouldn't be too concerned about moisture building up under the siding. That corner seam was never perfectly water tight to begin with.