This hack permits to restrict adding in the cart products in fixed quantity steps - so only 2 or multiples of 2 (4,6,8), and also to update the quantities in the cart using these steps.
Only one core file - the main virtuemart language file  - is modified, the other 4 files are template files... so it's on the edge... is not quite a core hack :)
How to install and use:
1. Edit the VM language file en-GB.com_virtuemart.ini found on /languages/en-GB folder by addintg the line found in the similarly named file in this package. Don't simply overwrite the original file!!! You must edit it.
Of course, if you use VirtueMart in other languages, you need to edit the respective language files too.
2. Unpack the files found in the components.zip package, and overwrite the files on your server with these. One of them is a CSS file, if you modified allready your template, or use a template different from the default one, you need to edit your original file accordingly.
The changes are easy to located, marked with comments.
3. Go to VirtueMart admin interface, and modify the value of minimum purchase amount to match your preferences. For example, if you want to sell vines in packs of 6 bottles, put 6 there.
The current version supports amounts from 1 to 14, for someone sligtly familiar with PHP/CEE is easu to extend to other values too!
4. Enjoy!