I had the same problem w/ my Phantom Labor:
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I primed the vinyl w/ Tamiya Fine White Primer, the painted it w/ Floquil Model Railroad paint. Engine Gray, I think (in retrospect, I wish I had used something darker).
It still chipped a little where the arm armor rubbed against it, but it otherwise worked nicely. As you can see, it flexes pretty well; just don't scrape it too much.
I've also used the Floquil Paints just by themselves, and they stick just about the same.