Blogs can be written from anywhere, on any computer, and in bits and pieces. You could write an update in an airport terminal, at your home, at your work, or in the park.
A vlog will require a quite location, a solid chunk of time, and in most cases a pre-written script, which could have probably just gone to your blog in the first place.
I suppose vlogs could put a more personal face on the startup, which may or may not help you connect with your customers. If you think it would help, you could create vlogs much more infrequently than the blogs, maybe just for large milestones in the company.
Interesting side note: Even though they broadcasted live 24/7 for a while, Justin.tv also had a blog. I'm not sure exactly why, but I'll bet that elad's point about blogs being indexed had something to do with it.