I try to always have at least a rough idea of the topic I want to talk about in my vlogs.
Of course, sometimes it winds up completely off-topic. My last one wound up being a ten-minute rant about needing to overhaul traffic laws and enforcement, when my intended topic was about remembering that everyone is the hero in their own narrative.
At least that way, if I don't take a wild left turn and ramble about some random topic for half an hour, I can still have something worth recording. Nobody wants to watch me ride around Rednecktopia and yell at crappy drivers. That's already a saturated market.