The term vanilla originates from the traditional standard flavor of ice cream, vanilla flavor. It's an adjective that describes the simplest version of something, without any optional extras.
Various computing-related things like software, hardware or algorithms are called vanilla when they are not customized, but remain in their original, basic form. In this usage context, the word vanilla is used synoymously to the words ordinary or standard:
Having no special or extra features; ordinary or standard.
There's even a website that builds upon this joke by attempting to market vanilla JS as a "very fast framework". Even though reminding developers that a lof of things can be done without using additional libraries, this is very confusing to beginners.