At one time 'vulgar' language was any language other than Latin.
The language of diplomacy was French and German was the preferred scientific language.
There was a lot of controversy over printing the Bible in vulgar language or providing church services in vulgar language. I don't know if they thought God only understood Latin or what. Jesus was a Jew, so I don't know why they thought God understood Latin instead of Hebrew.
Anyway, hearing people refer to "vulgar" language brings lots of thoughts to mind. I'm pretty sure I consider a lot of mealy-mouthed platitudes "vulgar."