I think it is a pretty kick-*** blogging software and has virtually every feature that you may want -- including trackbacks and rss. I think you will have no problems with google considering it a blog.
And quite frankly, what matters are the incoming links and the strength of your site rather than what google considers your site to be.
vBulletin is a software which is very limited by its out of date architecture...although it is the number one forum software out there, the technological potential of the software peaked several years ago. think internet explorer before firefox came along.
Some big VB forums are using these VB blogs, but I think not for SEO or as real blogs, but mostly for users who want to post their off-topic ramblings which do not go to any other forum sections.
The point is, do people actually use it? They (incl. myself as a user) may try it out for fun, but if less than say a few percent of your active members (excl. staff) actually use it (I mean by posting more than once or twice), what's point of installing and running it?