Give the manual another read, specifically R20. What makes you think that non-vex shaft collars would be legal (assuming it is for competition)? If it not for competition, then google “hardware stores near me”. QED.
R<20>c.
“Any commercially available nut, washer, standoff, and/or non-threaded spacer up to 2.5” (63.5mm) long which fits these screws.”
I think that this means it is up to the inspectors. I think the rules also said that smaller nonVex parts are acceptable as long as they don’t have any extra functions.
Vex wouldn’t care and an inspector probably wouldn’t notice but it would be a shame if we were up against each other in a finals match and you had to take off all of your shaft collars… point is i wouldnt risk it, just stick to the vex ones.