You're looking at more than just the cost of re barreling. You also have to make sure the larger cartridge feeds through the magazine successfully, plus the bolt face needs modifying. By the time you're done, you'd have most of the money into it to buy another rifle.
Too bad you need a lefty. A new Vanguard is around $500, and they come in 257 wby.
Switching your gun to 25-06 would be a slam dunk, just screw on the bbl and head space it. I doubt a deer is gonna know the difference between getting shot with one, vs the other. Both are gonna hurt! You just might need a little more hold over on the -06.
Didn't the Vanguard come in lefty a few years ago? Maybe you could find a used one?