no it's not Sintra I use a thicker plastick than Sintra. It's similar in composition but not as brittle
Also be sure that there is no paint on the plastic surfaces you are joining. I use Devcon Plastic Welder. They usually have it at Walmart.
First be sure that the pieces line up properly to your body.
For mine, I installed snaps to the armor pieces, and then I put a little paint on the ends of each one and pressed it against the vest on my body. It leaves drill spots for the snaps on the vest.
I cut a small hole in the fabric, and put the female snap in place, and then hammered it in.I am not quite sure I understand how you got the female end on your vest GCN.