If I were doing this in my basement, with the tools on hand, I would do the following.

Insert a long bolt through the tube, add a washer and nut, then tighten firmly.

Clamp the exposed end of the bolt into the soft jaws on the vice.

Use my tubing cutter to portion off the pieces I need.

Disassemble and de-burr.

Failing that, de-burr and lube the tube, then jamb a wooden dowel through it, and take that to the cutoff saw.