While it isn’t absolutely essential to have a microphone to vlog, it really does elevate the quality of your videos.
Having a microphone will provide you with top level audio for all of your vlogs, which is something a lot of viewers will appreciate.
It’s possible to vlog without a microphone if you keep the camera close and stick to quiet environments, but eventually you will want a microphone so as not to offend the audiophiles of YouTube.