There are just 2 longterm releases for Joomla:
Joomla 1.5.23 with support until April 2012
and from January 2012 there will be Joomla 1.8/2.5 as longterm release.
I am not a Virtuemart Developer, just a user, and all I can say is that I would not use VM 2 for a productive shop. I would say VM 2 is a public beta test version, where a lot of things do work, but a lot of necessary parts do not work. If VM 2.1 (the first stable release) will be available (maybe in Q4 of that year), Joomla 1.8 will not be ready. I am pretty shure if Joomla 1.8 becomes published, VM 2 will be ready too for that version. It is more important to remove bugs and continue the developement to make VM 2 to a stable public version, instead of trying to make VM 2 beta (which has bugs or not finished functions) compatible with the buggy Joomla 1.7 code.
For all those, who complain about lack of compatibility with Joomla 1.7 there are some other shopping carts for Joomla which say, that they work with J 1.7 or to write an own shopping cart solution.
Just my opinion!