The IVOA manages their Vocabulary Enhancement Proposals (VEP) through this repository. See Vocabularies 2, Section 5.2.1 for what a VEP is 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.
This is the index of current VEPs; VocInVO2 still has this on the IVOA Wiki at https://wiki.ivoa.net/twiki/bin/view/IVOA/VEPs; operationally, it's easier to keep this with the VEPs themselves.
VEP-018 Addition of product-type#hyper-dynamic-spectrum
VEP-019 Addition of ivoasem#ucd-syntax-code
VEP-020 Addition of product-type#event-bundle
VEP-002 Addition of datalink/core#documentation
VEP-004 Addition of datalink/core#counterpart
VEP-005 Addition of datalink/core#coderived
VEP-006: Definitions of datalink/core#calibration and its descendants.
VEP-007: Definition of datalink/core#detached-header.
VEP-010: Creation of datalink/core#package
VEP-011: Creation of datalink/core#thumbnail
VEP-013: date_role#Inspected
VEP-014: Addition of product-type#spatial-profile
VEP-017: Update of RefFrame#geo-app
VEP-016: Addition of date_role#ExportRequested
VEP-001: DataLink: Associated-DataSet branch
VEP-003: datalink/core#sibling
VEP-008: definition of product-type#dynamic-spectrum. (was procedurally unnecessary as product-type still was preliminary as a whole at that point).
VEP-012: date_role#Validated
VEP-015: Addition of relationship_type#References
VEP-009: Modification of datalink/core#progenitor