I've never understood why redweek charges people to respond to ads. Placing ads is a different story, and that's reasonable to charge for, but responding?
As for "is it safe"---redweek is really just a big classified advertisement board. In any transaction, you'll be dealing with an individual owner, and you'll have to decide how safe any particular transaction is. Importantly, you'll have to do that for VRBO as well. VillaDirect and AllStar provide a small extra layer of safety, becuase they manage the homes they market.
Things to watch out for when dealing with an individual owner: wanting payment with money orders or certified checks, asking for an unusal amount of money up front, an unwillingness to let you confirm your reservation with the resort, an owner that does not disclose a real street address or home phone number (as opposed to just a P.O. Box), etc.