Im gonna create an online shop using this template: https://www.gavick.com/joomla-templates/responsive-ecommerce-virtuemart-joomla-template,92 But it says that the template only supports VirtueMart 2.6 and I have to use Joomla 2.5... My question is... can't I install VM 2.6 in Joomla 3.X (?)
No, you can't.
For starters, the XML file uses the
<install> tags which were used in Joomla 1.5 but still compatible in 2.5, so trying to install it will result in:
JInstaller: :Install: Can't find Joomla XML setup file.
In addition to that, it's probably using older coding standards, which will result in PHP errors
I would simply suggest finding another template that's compatible with VirtueMart 3.x (Joomla 3.x)
According to this quite long tread from the GK Forums, the template has been updated with a Joomla 3/VM3-compatible version (just scroll down almost to the bottom):
Yes. Bike store form J3.x and VM3.x was released 2 days ago
That way you can install the J3-compatible template and run your site using the latest Joomla and VM versions (unless you've already found another solution to your problem :)