Joomla is the successor of Mambo (which is another CMS system) Joomla is fairly new but there seems to be a growing trend in developers and contributors creating modules and plugins for Joomla.
In my opinion Joomla is not going anywhere, it is here to stay and it just gets richer and richer and more easy to use.
My advise is that if you`re going to build your web site using joomla, that you use someone that has Joomla experience. Joomla can be very easy to modify and the template theme creation has to be done correct using the Joomla CMS stylesheet defaults. (you don`t need to use that).
As for virtuemart, I would suggest Jcafe!
these are all Open Source software you just need someone to create the templates for you.
Magento, is an Open Source e-commerce that is becoming very popular and has lots of neat features. The only down fall is that you need php5 and mysql5 to get it working. other than that its very easy to set up and publish.
Now back to the Joomla, I am currently over modfifying
a theme for a friend, just so you can see the power of Joomla. It is very simple but easy to use...
Web Developer / Owner / Consultant
When starting your own business the need to "know-how" is greater than money!