Well, I think everyone has their own talking style. And it's a very personal thing that makes you who you are. So, I don't suggest forcing enthusiasm; but rather, finding topics that you are naturally enthusiastic about. ofc, not everything needs to be entertainment-based either. In fact, most vlogs that I listen to come in the form of a search query anyway; where I just want the knowledge or a tutorial; and I appreciate a calm well-thought-out speaker, with dynamic kicks every once in a while.
So, I guess, if I was a vlogger, I would find out what topics are in demand-- but *also* lack in supply. It's the same way people make money by making websites: They find out what people are searching for; and see if there is a decent website about it already; if not, they make the site themselves to fill the demand. At that point, it 's just about tagging/titling your work towards the search queries, and inserting the information that they need to know. Most of the time, the website/vlog is just average-- but the content is there, and that's what matters because there aren't that many websites/vlogs about the topic to begin with.
Another approach is to find out what's already been done... and simply do it better. Maybe try doing a couple vlogs that way first, so you can see how others have become successful; then improve in areas that you think they may be lacking; or introduce a different viewpoint that wasn't addressed in the original. That way, you can submit a video response to the first vid, and piggy-back on it's popularity. (Although personally, I think the former approach is more valuable... but I guess it depends what you are trying to achieve as well