Also aluminum/steel siding will hold a coat of paint longer than wood, because it is not porous and does not expand/contract/flex with changes in humidity. The paint job on your metal siding might actually outlast vinyl siding, which deteriorates with UV radiation. I don't think 20-30 years is an unreasonable expectation. All for a shitload less work and money than a vinyl redo.
Wash with TSP and a good scrub brush / brillo pad prior to painting. Lighter colors and less glossy paints will hide dents better.