AP Style holds that you should spell out “versus” in ordinary speech and writing. For example,
- The proposal to revamp Social Security versus proposals to reform Social Security and Medicare at the same time.
In short expressions, however, the abbreviation “vs.” is permitted. For example,
- The issue of gun rights vs. public safety has been with us for some time.
For court cases, use “v:” for example,
- Marbury v. Madison was a landmark case.