I’ve been doing quite a bit of machining on acrylic lately. I don’t know how much useful advice I can give for flat parts (I’ve been focusing more on 3D carving), but for what it’s worth…
If I’m cutting out flat parts with a 1/8" end mill, I hold down the sheet with carpet tape (the kind that came with the Nomad) and take 0.5mm cuts at 10,000rpm and 1200-1500mm/min. Largely that’s because I use the machine in my not-very-soundproof apartment, but it also helps with keeping the work held down. I’ve noticed (from the smell) that deeper cuts heat up the acrylic enough to melt it, but I’m not sure that really matters; it might not be good for the cutter.
Holding down acrylic sheet is tricky. It’s flexible enough that clamping is no good, but rigid enough that as soon as the tape starts to let go, it lets go completely. I believe it helps a lot to make sure that every point is stuck down securely-- I use a bone folder (or a thumbnail) to get good adhesion on the part, with no bubbles, and I make sure the spoilboard is completely free of dust and maybe lightly sand it with 1200 grit before sticking the part.
The good thing about acrylic, though, is that even one small tab will do a lot to hold the part down. If I’m creating the toolpath manually (I use a Rhino script), I’ll arrange it so that I end up with one tab on each part, then just snip all those tabs with the end mill; probably one part in ten comes unstuck at that point, but the cutter has already moved on so it’s not a problem.
I usually finish the cut surfaces by flame polishing-- just run the hot part of a blowtorch flame over it (in a ventilated area), and you get a nice gloss finish in a couple of seconds. It’s a lot easier than a vapor bath, though I don’t know how it will affect your tolerances. I’d guess it puts a < 0.1mm radius on sharp corners. It’s fun, anyway.
ETA: to remove the tape, I just dump all the parts in the sink, clean off the residue with a drop of paint thinner, and wash the parts with dish detergent and water.