Thanks for the effort on this.... so I got it working. But it does not highlight in the manner where it is useable(as othree html5 does). Also, it breaks my vim-javascript highlighting.
I realize I am at a disadvantage here because I am not using a Microsoft product that Polymer is now geared for, but it would be nice if I could use vim with Polymer 3 as I did with Polymer 1.

Without this plugin with no breakage(as the JS should look):
