The best judge of this in my mind is someone who has both, the Mark V 26 inch barrel, and the vanguard 24 inch barrel. The person I spoke with said he loves his vanguard about as much as his mark v and velocity loss is only 50fps between his two rifles. That is still quite fast enough in my book to justify loading the 257 wby in a 24 inch barrel. I dont load to save money, so I dont care if it burns 10 more grains than a 25-06. I think weatherby is smart to release their propriority cartridges in the Vanguard. They have several custom stocks you can upgrade to, and what if, they began offering a custom barrel upgrade also. They could really have something with that.
Myself, I suspect I will really enjoy shooting Roy Weatherby's favorite cartridge. The most important thing for you to do is what you want to do.

Perhaps I would re barrel a few years down the road, it's sitting in a rifle that definately can handle the weatherby loads.

JC, did you ever check to see what the weatherby factory ammo velocity was coming out of your 24 inch tube?
Just curious if you did that.