Hi @ALCArobotics, welcome to the VEX Forum!
Any question that asks “is it legal” should be answered by consulting the game manual. In this case the relevant rule is <R6d>:
(emphasis mine)
So, you can only use rubber bands if they are:
- Purchased from VEX as part of the EDR/V5 product line, or
- The same size as rubber bands that VEX sells as part of the V5/EDR product line.
(can you use rubber bands made from different materials? see this thread)
The question is then, which sizes of rubber band does VEX sell as part of the V5/EDR product line? Looking at this page, we see that “VEX-brand rubber bands” are only available in #32 and #64 sizes; thus, those are the only sizes legal for use in VRC.