... one more - using vmuikit 3.30 / vm 3.0.18 / joomla 3.6.5
on product details page. If I enter the quantity manually, I get an "undefined" in the quantity field. Hit enter on keyboard, nothing happens, click "+" and I get a error message " ... you can only buy that product in the multiple of 1... " (loose translation) ... see screen.
can you make sure that you have latest virtuemart ? because you will see quantity "undefined" error if virtuemart js files not loading in cart page. Please send us ftp if you need further help. i will check your files