I installed VM 2.6.6 earlier this year (with Joomla 2.5) and have done all of my design work using it. Now that I'm several versions behind the latest version, I have to ask, what big changes are there in the new version that I'm missing in 2.6.6 (besides te security issue)?
My big fear is upgrading and then discovering that most of my work has to be redone. Are the VM3 pages still the same, layout and coding wise?
You could upate a vm1.9.8 directly to vm2.6.14.
Should be the same for vm3, just install it over your old installation. Read here for more info http://docs.virtuemart.net/tutorials/installation-migration-upgrade/198-upgrade-virtuemart-2-to-virtuemart-3.html
The coding is much improved and there are changes in templating, with sublayouts now being used. IMHO the templating system is a lot better.
Thanks! Another question: if I'm using template overrides, is there a reason for me to worry that VM3 will not render well, or is the templating system the same (vis. overrides)?
Overrides can still be used, but the improved templating system uses sublayouts (which can also be overriden). There may be come code changes needed, but some people have already found that the improved display in VM3 is suitable for their use and have discarded old overrides.