Rather than "Drill and Countersink", if possible I would drill and tap the jaws so that the retaining bolts are outside the area used for clamping.
This is how the jaws are secured to the Vertex K Series Vices.
To ensure that everything stays square, it might be worth clocking the vice so that it is aligned correctly, along the table, before fitting the jaws.
The take just a light skim with an End Mill so that the new gauge plate jaws sit on accurately aligned face, when is finally fitted.
Why "Finally fitted"? because once there is an accurately aligned face, it can be used as a datum to mill the new jaws to size, before fitting. You might have to be very gentle milling the ends of the jaws when they are clamped vertical, because of the flexibility of the metal protruding from the vice.
It is worth having the ends and top of the jaws truly horizontal and vertical, so that they can be used as a datum plane for setting later work. .
The top and bottom of the jaws will be easier, since they can be secured to the vice body for machining..
If you wanted to add a little complication, you mill a Vee slot (Horizontal and / or Vertical ) in the fixed jaw as an aid to gripping round work.
For this you might need to make up a temporary fixed jaw from mild steel for these operations.
HTH
Howard