isn't it a requirement that they all have life jackets?
Yes, and they do. In the 738's the lifejackets are overhead, not under the seats. They can't be stolen from how I understand it.
valujet was wiped out in the US by a crash and ryanair's traffic and stock value would surely plummet in the event of one.
ValuJet had a history of accidents, faults and god knows what - nothing like Ryanair in the least.