2009/8/10 Nathan nawrich@gmail.com
I think "Vector" is the skin, and Acai is the usability initiative. A remember reading a blog post where Vector was described as the first element in Acai.
Acai is just the codename for the first release of the usability initiative. "Vector" is indeed the skin part of this, and there is also an enhanced editing toolbar which can be enabled in Special:Preferences (Editing tab). The next release will be called "Babaco", and is in the design stage: ( http://usability.wikimedia.org/wiki/Babaco_Designs). The plan is for four releases in total, all named after tropical fruits. ( http://usability.wikimedia.org/wiki/Releases)
I particularly like the left-hand navbar links to featured content.
Monobook has this as well I believe.
Pete / the wub