I would add that it would likely depend on when we're talking about. Vulgar Latin (a problematic term) wasn't a static language. At earlier times, it would have seemed that it was just a less formal or lower-class register of Latin, but in later times, they might not have been mutually comprehensible. When and where this break occurred has been the subject of a lot of research. Current thinking seems to push the date later and later, in that there is some reason to think that even as late as the seventh or eight century, people regarded Classical Latin as just the written form of the spoken idiom, and possibly pronounced it as such. In other words, they would have written, for example fidem but might have pronounced it as if written feidhe or something like it, assuming this theory is correct.