The native language of a country or place. A vernacular term that has achieved widespread recognition may be used without explanation if appropriate in the context. Terms not widely known should be explained when used. Also, see Regional-Specific Terminology.

Spell it out in ordinary speech and writing: The proposal to revamp Medicare versus proposals to reform Medicare and Medicaid at the same time… However, in short expressions the abbreviation vs. is permitted: The issue of guns vs. butter has long been with us.
Note: For court cases, use v.: Marbury v. Madison

Use two words: vice principal, vice president. Capitalize or lowercase following the rules that apply to president. See Capitalization, president.  

voice mail
Two words. Avoid using as a verb.