I've suggested that [vulgarity] should be a synonym for [offensive language].
The vulgarity tag's description is
Use this tag for questions that relate to questioning whether certain words are taboo or if your question relates to the level of appropriateness of a word or its possibility to offend in certain situations and contexts.
The offensive-language tag's description is
This tag is for questions about offensive language. It is for questions about words or phrases that could be considered offensive. If reason of offensiveness is belittling or painting a negative light instead of 'just offending' CONSIDER using the tag PEJORATIVE-LANGUAGE.
There is a difference between using language which could be considered offensive and asking whether certain words are taboo. With the former one knows that there are people who do consider the language offensive, even if there are also those who do not. The latter suggests that you think the words might be offensive, but there is no certainty that anyone actually finds them so.
There is enough of a difference there for these not to be synonyms.