Paul Lloyd's 1979 paper "On the definition of Vulgar Latin" gives an amusing 13-item list regarding how the term has been actually used:
1. the direct descendant of Classical Latin: Classical Latin > Vulgar Latin
2. a direct descendant of Classical Latin, emphasizing its parallel existence to Ecclesiastical (or Late) Latin
3. a direct descendant of Old Latin, coexisting with Classical Latin
4. equivalent to "popular" Latin, the Latin of the average people, particularly inferior social classes
5. lower-class Latin, spoken by plebeians
6. "uneducated" Latin
7. middle-class Latin, as opposed to both Classical Latin, and low-class and rural Latin (sermo plebeius, sermo rusticus)
8. ancient colloquial slang, therefore limiting the term to lexical concerns
9. spoken Latin of any sort, as opposed to written Latin
10. "vanished" Latin, i.e. terms reconstructible from Romance (e.g. *excorrigere > Spanish escurrir (in the specific sense of 'to go out to say goodbye'), Old Italian scorgere 'to escort, guide'), therefore limiting the term to lexical concerns
11. the Latin of the non-Roman inhabitants of the Roman Empire
12. Latin reconstructed from Romance languages (and not just in terms of the lexicon)
13. "diasystem" Latin, "a collective construct formed by combining many varieties of Latin, but designating the actual speech to no one"
A quote from the paper:
Since almost everyone who has ever taken the trouble to examine the many definitions that have been devised for "Vulgar Latin" has come to the conclusion that it is inherently ambiguous and contradictory, its persistence in scholarly writings is truly amazing.