Using PEX tubing to carry vaccuum

You might have to be an early adopter. Obviously the big advantage of PEX is that you need very few joints–in your situation maybe only 2. There are a variety of PEX joint techniques ranging from barbed fittings using the PEX’s “memory” to seal to metal crimp rings and a few others. If you are operating at room temperature I bet you can make it work–the metal crimp rings may be the most robust in your application. I say this from construction experience not lab.