Switch to the Hornady 100gr "Interlok". Thats what the factory loads in the Weatherby ammo. I love the Sierra's in my .257Robt, but they are a tad "soft" for the Weatherby. (I too have a Vanguard that not a "MOA" model, does shoot moa. I killed a Doe with the load you are using (IMR4350 @ 63.5gr) and yes, it chrono's the same as your gun. I used the Hornady 100gr Spt. I shot a doe @ ~90yds. Bullet was recovered under the hide on the far side. Double lung and heart shot. Deer ran about 100yds. Typical. (I shot a doe in my back yard yesterday w/150gr Nosler BallisticTip @ 2,800fps from my 7mm08. The double lung shot 70lb doe ran ~100yd half circle before dropping. Typical for particular shot placement). Recovered bullet from doe shot w/.257wby and 100gr Interlok weighed 63.5gr. The interlok feature worked as advertised (again. I recovered a 117gr BtSpt Interlok from a deer shot lengthwise in 1993. bullet weighed 97.0gr and perfect mushroom- 240lb deer, bang flop @ 270yds from .257Robt @ m/v of 2,800fps.).
I also suggest you try the Nosler 100gr Partition, as well as the Swift A-frame. Either of these will give accuracy near equal to the Sierra's and superior penetration.
If you still have "issues" with the lighter bullets, move up to the 117gr Sierra's or 117gr Interloks. The Interlok will perform very well from the .257Weatherby.
I use a non-cannister powder called WC-860. It is the mil-spec version of Hodgdon's discontinued H870. With 76.0gr I'm getting 3,200fps and MOA accuracy.
A final option is the Nosler 110gr Accubond. I'd try these with the IMR4350 you have at 58.0-60.0gr. Likewise, Reloader25 @ 70.0gr will run them to about 3,300fps. If they are accurate from your gun, that's your load........ I'm using this with either a 115gr Berger or 117gr Sierra or Hornady Boat-tail Soft point.
I strongly suggest you try the 117gr BtSpt Hornady first as it is an "interlok" bullet and holds together like a Nosler Partiton without the 3x cost.
I bought 500 "blem" Hornady's (100gr Spt) and use it over 74.0gr of RL25 for 3,700fps. It smacks deer at over 500yds with authority....... But if I don't hit heavy bone, they will run a ways....