All of the molded plastic stocks flex a bit, regardless of brand.
This is why some manufacturers install aluminum bedding blocks and longer in their firearms among other reasons.
The average user of todays modern firearm will never (imo) outlive the accuracy and functionality of their firearm.
Of course that statement has some limitations as does the number of rounds down the pipe and claims of the average user.
You will know in very short time just how accurate that Wby. chambered Vanguard is and how much fun it will be to shoot and harvest game with.
The two .257 Wby's I have shot/shoot duplicate the factory target and a little tighter with 100grn bullets and the Leupold VXIII-3-9 and that is good enough for me and that is all that I expect for this combo.
There are others who can and do tailor their ammo and get better results and then there iare the guys who do evven better with a different rifle and chambering all together.
Just remember

"nothing shoots flatter, hits harder or is more accurate" than a Weatherby. A key component in that claim is the ballistic performance of Weatherby Magnum ammunition... end of commercial pimping .
Seriously, there is a rush when one busts that cap knowing that the 275 yrd target is going to drop when the crosshairs are placed in the boiler room .
Tight Groups and Good Eating !
Rob