I'd recommend always writing a script, whatever your type of video. (You don't have to look at the script whilst recording, but it helps you get your thoughts together. Maybe you could sign them and auction them for charity or something if you ever get YouTube famous...)
Then again, my videos are wierd.
I have a very strange way of vlogging.
I almost always write scripts - first I jot down the things I want to mention, in tiny writing at the top of an A4 (or A5) sheet of lined paper, then I write down exactly what I'm going to say, word-for-word, until I exhaust all the topics I jotted at the beginning.
When it comes to recording, I memorize a sentence or two at a time and deliver them to the camera, although what I say usually differs in wording from what's on the paper, and sometimes I expand upon it, going into details which would have taken too long to write on paper.
This method means there are a lot of cuts in my videos, and it may make them seem less natural, too. On the other hand, many of my videos aren't vlogs, as such, but 'shows' that require scripting (ocasionally, they're sketches).
It also ensures I never forget to mention something, or pause for too long (I cut out the bits where I look at the script - over 50% of the source footage for each video shows me reading what to say next)
I do occasionally improvise a video entirely, by simply talking to the camera about things I need to talk about. I normally only do this when I'm documenting an event, or I haven't had time to write a script. It gets mixed results...
My scripts are usually about two sides of A4 per five minutes. I shred and recycle them afterwards. Don't really know why!
