William Duffield
Following up on Bill's and Howard's suggestions,

You could replace the mounting board under the PC DT jig with a sturdy box construction to lift the jaw to a more appropriate height for sawing, while decreasing the vibration.

Most Nicholson style benches (with the wide front apron) have more dog holes, in a zig-zag pattern, to offer more vertical work holding options, even though they were not necessarily used with a Moxon vise. I see that one of your holdfasts is very close to being ineffective because the closest dog hole in your sparse dog hole pattern is almost too far from the corner of your work piece. If the Moxon jaw were closer to the joint you are sawing, there would be less need for the bottom left holdfast.

I hope you don't realize one disadvantage of the PC DT jig, which is the damage it could do to your saw teeth if you oversaw or incorrectly place the work piece.

I can't tell for sure from your photo, but if your front apron started with a plank less than 8/4 thick and made of soft pine, the dog holes will soon start to be wallowed out by the force of setting and removing holdfasts, and the holdfasts will eventually start to loosen their grip. There's no need to replace it right away, just be aware of the possibility when and if the holdfasts seem to be losing their grip.