Have experienced some quirks with the Vanguard synthetic stocks over the years, even after "bedding" the action and free-floating the barrel. This may be heresy, but haven't experienced the same thing with bedded/floated walnut stocks. In fact by far the most accurate Vanguard I've ever owned was a .257 Weatherby Sporter, with a traditional Weatherby-style stock. Put three shots into 1-1/2"--at 300 yards.
The trigger change may have something to do with it. Often Timneys do take up more room, and the trigger might be bumping the stock, as Drover suggested.
I have never been able to install a Timney without grinding the stock a little bit. I have as well seen a walnut stocked Vanguard in 257 and another in 300 wby shoot tiny little groups with factory ammo.
