We have 3 older Mark V's in our family that we picked up back in the early 90's. Those are great rifles with a quality barrel and muzzel brake. They have the recoil of a 30/06. My 80 year old dad could shoot a box of 20 off the bench with the brake but couldn't shoot it at all without the muzzel brake.
You need to checkout the current Mark V's and see where they are manufactured and assembled as well as what type quality steel the barrels on the Mark V's are manufactured from and compare that to the Vanguard Spec's. So many rifles have parts manufactured in different countries the rifle is assembled in a different country etc... Make sure they have the same bolt design. I was told that the Vangaurd back a ways had a totally different bolt action than the Mark V so check that out... They are not the same as in the old days. Then make your choice. We love our 300 Weatherby's. If your going to work up a load try Nosler 180gr. Accubonds pushed by IMR 7828 powder at 84.5gr and a Fed215 primer. Works great in all our 300WBY's.
Obviously the Mark V is a step or two up from the Vanguard in quality and price or they wouldn't have made the Vanguard in the first place. They have to access a certain price range customer market to stay in the rifle selling game. If your pocket book allows you to purchase the Mark V you will never be sorry. That rifle can be handed down through the generations and will always be a quality rifle even as all manufactures find ways to put less quality into their rifles as costs of manufacturing and materials continue to sky rocket.
Good luck choosing your rifle take the time and get on the Weatherby website and to some research on craftmanship, materials, components and compare then make your choice.