If you're flying over water, the plane is required to have inflatable life vests available to each passenger. If you don't trust the airlines' life vests and want to bring your own, then - paradoxically - you may be in violation of the airline regs.

There was a thread here - or maybe an article by Doug - about how ferry pilots are flying long distances over water with no life raft, because the life rafts CO2 canisters are classed as HAZMAT and the cost of shipping the raft back home after the ferry flight would be more than the pilot got paid for making the flight. I forget the details, but I'm sure someone else can point to the source. (I don't know if this would apply to a CO2 life vest - obviously, though, it doesn't affect the ones that the airlines are approved to carry on each flight.)

If you're talking about a non-commercial flight, that would be different, I suppose. (After all, there's nothing preventing a ferry pilot from taking a life raft along, other than the expense of getting it back afterwards.)