Vacuum veneering question

I've done some hammer veneering but this is the fist time I'm using a vacuum bag. The question I have is how do you keep veneer from shifting on the substrate?

I'm building a walnut sofa table and originally I hadn't intended to veneer the front aprons and drawer. But then I came across a piece of walnut that screamed out to be used to veneer the front. I understand that normally you cut the substrate larger than the veneer and trim later but the problem is that the parts are already cut and the joinery completed so the veneer will need to be cut to the size of the parts. I'd prefer not starting over if possible, although it I guess it could be done. Is there a way to insure the veneer doesn't shift after it goes into the bag? The veneer is shop sawn walnut, probably 3/32" going onto solid walnut substrate.

Thanks for all advice.

Cliff

Ordinarily I cut the veneer larger than the substrate, then trim afterward.
If you absolutely can't do that, shooting a couple 23 gauge pins thru the veneer should hold it. If you have a 23g pinner

I have used packing tape to keep commercial veneer in position during vacuum pressing. There won't be much (if any) lateral pressure on your materials so the tendency to shift should be minimal. One problem with this approach is that squeeze out can spread under the tape rather than beading up nicely, so you might want to prefinished the edges of your substrate.

I use blue painters tape.