FWIW I have had plastic rings on 4 sets of wheels & never once had a problem with them. My money is that one or more of your tires doesn't balance out. Call Tire Rack & find out what is an acceptable road force number for your tires.
I have the info, waiting on the test tomorrow afternoon The wheels were from tirerack tires from a local outfit. Either way, someone is gonna fix it. I've found out it can go either way, no issues or issues, with hub centric rings.
Yeah plastic or metal hub centric rings should not matter at all. My money is on a balancing issue also. Good Luck