An online site with information on barrel manufacture had images of certain problems that can pop up unexpectedly with button rifled barrels.
One of these was when the initial boring left microscopic concentric striations the full length of the bore.
When the button is pushed is pushed through the tips of the striations are all pushed over in the same direction.
If turned and chambered so bullet travel is in the same direction as the button pushed through the bore then theres no problem. If chambered from the other end then these turned down striations act like microscopic file teeth and will grind away at the bullet jacket. Copper fouling is then many times worse than it should be, and the surface won't smooth out by simply firing a lot of rounds.
If this is the case with this barrel lead lapping or fire lapping may cure the problem. Lead lapping is prefered.
First source I looked at says the Weatherby Vanguard has a hammer forged barrel, the Mark V has a button rifled barrel.
If your barrel is hammer forged, and you wish to lap the bore, remember that the bore surface of hammer forged barrels normally have a dusty appearance that may not go away if lapped. They warn that one should never try to get a bright shiny look to the bore of a hammer forged barrel, and over cleaning in trying to get a shiney bore can wear the rifling out.
Heres a site with some interesting images.
Note the Savage button rifled bore has very visible striations, apparently this bore is properly oriented, putting the turned down edges towards the muzzle. little or no copper fouling showing.
The Tikka stainless steel hammer forged bore looks very smooth, yet has collected more copper fouling.
This may be due to the alloy used. It was found that when autopistols are made using the same stainless steel alloy for both slide and frame that these were subject to galling. When slightly different alloys were used the galling problems went away.
Could be that the specific alloy used has an afinity for some bullet jacket alloys, and may not have the same fouling problem with bullet jackets of another alloy.
Molycoated bullets might be an option.