I doubt you will get enough holding power.
2.5" diameter is 4.91 square inches.
Full vacuum, which is not possible, is 14.7 pounds per square inch.
You will have less than 72 pounds holding your part down, it will certainly spin and very likely will lift out under the force of the cutter.
Side clamps are not out, you have 0.5" of space to clamp on the side; top clamps are out.
Do you have a decent sized vise? Make yourself some custom jaws with a 2.5" diameter hole about 0.45" deep and you will have plenty of holding power.
When you want to scale up for high volume start another thread and I will suggest how you can make a multiple holder using air clamping.