We've been using a 2 flute 1/4" down spiral (1" CL) for most all acrylic that is pre laminated with vinyl. The down spiral keeps the vinyl down and cuts clean at the edges. We don't flame polish, I tried a few pieces once and didn't have good success even without the vinyl.
I chuck up my bits so that just the cutter is exposed. Seems to keep the chatter down and makes a smooth edge by elimination of bit deflection. Unless we have the exact same machine and spindle, I'd guess that your speeds and feeds will be different. Probably experiment a little to dial yours in.