When the RC flys the "Y" flag, the rules require that sailors do actually wear a PFD. In that case, I would expect that wearing a non-approved PFD would be a protestable offense.
While you can protest anything, winning the protest is another matter. RC's rarely fly the flag and even if they do, RRS 40 says:
40 PERSONAL FLOTATION DEVICES
When flag Y is displayed with one sound before or with the warning signal, competitors shall wear personal flotation devices, except briefly while changing or adjusting clothing or personal equipment. Wet suits and dry suits are not personal flotation devices.
Race Signals "Y" - Wear a personal flotation device.
The US Sailing Prescription to RRS 40:
US SAILING prescribes that every boat shall carry life-saving equipment conforming to government regulations that apply in the racing area.
A Zhik vest is a CE-approved personal flotation device. It's just not a USCG-approved one.
You would not win that protest. (Assuming the protestee was carrying a USCG-approved PFD.)