Let's clear up a few things here...
There are no wind-based requirements to wear life jackets in the Hobie class rules or standard SIs. Haven't been for the last 12 years that I've been involved.
As Harry said, the flag is code flag Y. When it is flying, per the RRS, wearing life jackets is required.
As Mark mentioned, some other classes, especially for Youth, add mandatory life jacket use into the SIs.
Now that we have the basics cleared up, let's get away from this idea that somehow, the RC takes on more responsibility. I can't believe we need another thread on this, let me go get a hot poker for my eye...
If you really think the RC will be liable, read the rule book. The RC is one party at the regatta. We are NOT there to be the police. There are also other competitors out there. It isn't possible for the RC to see everything people are doing on the water.
If the Y flag is flying and a competitor chooses to not wear a life jacket, it is the sailor's responsibility, as they are breaking a rule. Hey, there's something new in America, let's have people take responsibility for their own actions!
I spent a great number of years working with PU. He, and many other wise PROs have taught me one thing if nothing else: This is a self-policing sport. If you want to spend your time (as RC) in the protest room instead of the bar, that's always your option. But, if the sailors aren't going to police themselves, it's not our job to go out there and find things to keep the jury busy.
Obviously, if you (as RC) witness something blatant, and no competitors were in the area to see it, and you have rock-solid proof, it's in everyone's best interest to approach the sailor and ask them to resign. If they fail, then file a protest from the RC. But, that is not what we're dicsussing here...
I rarely recall an instance of seeing the Y flag up at a major regatta. If I have, it's usually at the OA's request. I don't know too many PROs who think it's really something that is necessary. We have enough to do to get racing off on time and on a square course.
Mike