Why not use the store truck bed liner or other vinyl specific spray paint; e.g.
Similar to epoxy that has a resin and hardener, Line-X consists of whatever the Line-x stuff is and a hardener. The Line-X shops use a special spray nozzle that combines the two materials when sprayed and then it cures on the surface of whatever is being coated. Sometimes they use a primer if the material being coated isn't Line-X friendly. As a truck bed liner it is tough stuff that is very tough and essentially doesn't wear. For example, I knocked a system case off of one of my Ks on a truck bumper while filtering at a stoplight in San Francisco. It did not get scratched at all.
I once tried rattle can truck bed liner on a K100RS belly pan. Even though I used the right primer for it it didn't last very long.
I have never tried vinyl paint on knee pads so I cannot comment on how well that works. I can only comment on my satisfaction with Line-X.
I have a lot of Line-X on my K1100RS (mirrors, belly pan, front fender, gas cap, knee pads, tail cowl handles, system cases) and I put a "racing stripe" of it on the bellypan of a K75S where gravel usually beats up those belly pans. I also put in on LT mirrors and the top case lid since those like to get scraped up. It has all performed great.
Did I mention that I'm a big fan of Line-X?