I do a fair amount of CNC work in support of my guitar builds but have not ventured into vacuum hold-down, though I have considered it. So take my comments on your hold-down issues as wild speculations based on no direct experience.
A few thoughts:
-- I've found it challenging to get an entire top or back to lay flat on my spoilboard. Might that be true for your vacuum hold-down. How do you get the seal to be "good" at all points? (even a quarter inch of blue foam that's not in firm contact with the guitar top (or back) will be a big enough leak that you'll never get suction.
-- Can you put another piece of MDF on top of the fixture and get good sealing? If not you'll never get a guitar top to "stick." If you can, can you put a guitar top on the fixture with the vacuum pump off, then lay a sheet of MDF on top of the guitar top, then turn on the vacuum pump, wait a moment, and then remove the MDF sheet.
-- Looking at your fixture, I'm concerned about details like the sharp edges of the blue foam where it bends around the corners at the verticals of the "D." That corner won't compress nearly as easily as the straight line or the gentle curve of the D. That means that the guitar top (or bottom) may rest on the folded corner and not compress the rest of the foam. And that means leaks.
-- Can you (dare you?) put soapy water or colored water on the blue foam, put your material on the fixture, turn on the vacuum, wait a bit, turn off the vacuum, remove the material, and see, from the pattern of the water, where the vacuum leaks are? As an engineer, I can't imagine that any significant volume of air is coming through the shellac-sealed MDF base. Of course, if your leaks are massive (and I expect that they are), this experiment will just make a mess and not tell you much.
And a question and a few of observations.:
-- how do you expect to use the fixture? For inlays or rosette channels, it should work fine (once the hold-down issues are resolved). But for soundholes, you'd best have very accurate (and uniform) thickness measurements for the guitar top because the vacuum fixture doesn't allow a spoilboard. I have had tops whose thickness was specified by LMI and other vendors, tops whose thickness was set by using a friend's full-width thickness sander, and tops whose thickness was set by me using planes, scrapers, and sandpaper. None were particularly uniform (the tops that I thicknessed were the least uniform of all; lack of uniform thickness seemed to have no negative impact on the tone of the completed guitar.)
-- I believe that leaks in a planar vacuum system come, almost entirely from the peripheral seal. The area under vacuum is not particularly important. In your fixture, the "D" foam in the lower bout and the ovalish shape in the upper bout may be causing some of your problems. If you want them as spacers to keep the soundboard flat from edge to edge, I'd suggest using only short pieces of foam in the straight and gently curving sections of your upper and lower bout shapes. It's all under vacuum. See if you can pull a vacuum with those foam sections removed.
-- I usually cut both the rosette channel and the soundhole after gluing a second layer to the back of the soundboard. This layer only extends a bit past the rosette. I don't know how that could be accommodated by a vacuum fixture.
-- I find that cutting on the soundboard is an important use of CNC in lutherie, but is probably only about 10% of the CNC work that goes into my guitar builds. I also use the CNC for cutting headstocks and headplates (some with bas-relief carvings), bridges, slotting fretboards (including fan fret), cutting bas-relief tail wedges, cutting mortices for bolt-on necks, and cutting nuts.
-- I use double-sided tape for most hold downs.
Just some ramblings, but they might spark some ideas for you. Your fixture is a beautiful piece of work. The next step is getting it to work. Best wishes.
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