The IVOA manages their Vocabulary Enhancement Proposals (VEP) through this repository. See Vocabularies 2, Section 5.2.1 for what a VEP ist and why you may want to create one.
Before starting a new VEP, it is probably a good idea to discuss your idea on the IVOA semantics mailing list. Once you are ready to enter the formal VEP process, please follow Appendix C.1 for Vocabularies in the VO.
VEPs can also be managed outside of github if authors have privacy (or other) concerns. Contact the chairs of the IVOA Semantics WG for more information.