I used regular the 'hook' side of sticky-back Industrial Velcro for the back of the armor. Once that stuff sits on there for a few days, it does NOT come off!
For the vest, I used the 'loop' side of sew-on version of Industrial Velcro.
The good thing is that you get a strong hold, but you waste as much Velcro as you use. My machine doesn't like sewing the 'hook' side of regular Velcro (too many tension snags), so I don't even use it. I end up throwing all the scraps away.