I understand your situation. .300 Weatherby + Vanguard = one of the worst combinations ever conceived.
The cheapest solution would be a Boyd's laminate stock
, and a good recoil pad. (The Boyd's buttplate isn't going to work for .300 Weatherby.) The Vanguards don't have a 'magnum' length action, so the long action version is what you'd be looking at. However, they don't offer any thumbhole variants for the Vanguard.
Just a rough estimate... I'd say the Boyd's stock should add at least 18-20 ounces to the rifle, if you have the synthetic stock. ...maybe 10-13 ounces, if you have the "hardwood" stock.
You could also consider a recoil reducer/decelerator in the stock. But... they usually aren't cheap, and will require a visit to a gunsmith with experience installing them.