If the fares are running $400-500 there's a very high likelihood that there won't be any reward availability. Generally the only time you can use miles to avoid an expensive ticket is when the flight load is light (and lots of fare bucket availability) but you missed the advance purchase requirements of the cheap fares. However, these days the airlines will likely charge you an expedite fee for the close-in reward booking, so they get you anyway. (Not to mention there's usually a fee to redeposit miles from a cancelled award, as you suggest).
In mid-July the cheapest published fare right now for NYC-LAS is ~$360 r/t on CO or HP (Q class for both). Not sure about JetBlue though. If actual itineraries are pricing meaningfully higher, it probably means the flights are filling up and the cheap fare classes are sold out. They can always be released as departure approaches if seats aren't selling well, and it's up to you whether to gamble that this will be the case (hey, you are going to Vegas, why not get a head start!). But the convention market there is hot right now, partially due to New Orleans, and depending on the schedule, flights can fill up quickly!