I too had trouble getting BVE4 to work with Vista until I tried to install version 2 (not as many features), then suddenly BVE4 magically decided it wanted to suddenly work as a result! (Made me quite happy that did!).
Release date is unknown - I believe the problems with Vista are due to issues with DirectX and the fact version 10 doesn't back support version 8 (which is what I think BVE4 uses). Regarding demos - put bluntly, I'm afraid not - the program is coded as a whole and will be released as a whole.
You could try to install BVE2 again as I did and see if that helps at all - details for that can be found at
http://www.railforums.co.uk/showpost.php?p=176152&postcount=132
May be worth running BVE2 once after installation (and
before the steps in the linked post) though in case that changes any files which help BVE4 to work.
Please check your PMs as to getting BVE2
