Just tried VM 2.0.26 on latest version of Joomla 2.5 and Joomlart T3 responsive template. Reading so much good reviews supported me in trying VM first. Unfortunately I'm really not that positive about the first results. Shopping Cart module refreshing problems. Responsiveness not working correctly anymore due to unwanted code generated by show shopping cart. Tried a lot of things found in different forums to resolve this issues. Enabling/disabling JS. Enabling/disabling caching ... etc. but without any positive results. I stopped trying and will consider another shopping cart. Really waste of time.
We are sorry for your experience, but as you said yourself, you used a T3 responsive template. It is very likely that you just had a conflict with a double loaded jQuery as described in our FAQ, which leads to various errors. VM is a component and not responsible for responsiveness, that is done by the template. If you want to test VM, either disable the 3rd party template or buy some compatible to vm.