I don't script my vlogs per se, but I usually write down a few points I want to make so I stay on track. When I get an idea for a video, I usually develop it in my mind for a little while (in the shower, while driving, etc.) and write down the major points I want to make.
But it depends on the video too. Sometimes, I have one big idea and just improv it through the whole video, while for other topics, I write down a lot of notes.
When I don't have a clear idea of what I'm going to do, editing takes a lot more time. I have more clip rearranging to do, more "blank stare at the camera" segments to skip through, and it's a lot more work to make it a cohesive video as a whole.
I can't imagine totally writing a script for a vlog. I'm not a big fan of writing (which is kind of ironic since I've practically written a novel here) so I would get sick of vlogging really quickly. But it's all about what works for you!