I'd put a couple of coats of car wax on it, then spray it with some Boeshield or similar protectant. If it's got an exposed lead screw, you'll want to grease that well.
A cover is a pretty good idea too.
They do hold up better than expected... I have a POS vise on a welding table I bought 2 years ago. My intention was to get it inside my barn, but I haven't found room for it so it's been sitting outside in the rain and snow. The vise is a little rusty but I literally haven't touched it since spraying some WD-40 on it when I parked it outside.