The term vanilla is sometimes used by members of the BDSM community to describe a sexual behavior that isn't considered kinky or doesn't refer to a specific fetish. Some classic examples of that would include sex in more traditional positions (such as missionary or cow girl), without any props or power dynamics involved. Some people use this term in a derogatory way, as a synonym for "boring", which we do not recommend doing. There's nothing wrong with vanilla sex, and you can be into BDSM but still enjoy vanilla, too. Some even like doing it as aftercare for a BDSM session.
Drawing a line to define which sexual practices are vanilla and which are not can be nigh impossible, since it's such a subjective concept. Is anal sex vanilla? Oral sex? What about the doggy style position? Different people will have varying views on each topic, based on their own experiences and surrounding cultures.
The take away from that is how devoid of purpose the term "vanilla" may actually be. If you think something is vanilla, while others do not, that's okay. Don't kink shame (or, in this case, vanilla shame?) others. If you like doing something you yourself consider as vanilla, that's also perfectly fine. Do remember, however, to clearly communicate with your partner, keeping in mind that what seems natural and simple to you might be something they are still exploring. As long as everyone is on the same page, everything will be fine!