What factors encouraged the use of the vernacular in literature in renaissance society?

Well for starters not everybody could interpret literature for themselves back then so vernacular is nothing but a common language so that everybody could understand what was written. This really came into effect later in history when people started to seperate from the churches and read what their god said for themselves instead of being told how to live by a priest who abused his power.

the american revolution was by the middle and upper class, and the french was by the lower and middle class.

during the middle ages bibles and most christian literature was written in latin i think that’s the answer you are looking for