Hi,
Having "undefined" value in the quantity box is a known issue and it occurs only if the "custom field, stocable variant" and VM template override used.
See this forum thread:
forum.virtuemart.net/index.php?topic=108768.0
Some people who are not using our VM Template are suffering about the same exact issue. I hope this issue will be solved in the next releases of Virtuemart, because the Core VM File must be edited which is not allowed to template override, so we (as template developers) are not able to solve it.
You can solve it by little modification though;
Open this PHP file:
ROOT/plugins/vmcustom/stockable/stockable.php
Find this line (in 403th line):
<input type="text" class="quantity-input js-recalculate" name="quantity[]" value="\' + quantity + \'" />
and change it with this:
<input type="text" class="quantity-input js-recalculate" name="quantity[]" value="1" />
(basically you are changing only
value="\' + quantity + \'" with
value="1" )
This fix should work for you. Try and let me know if doesn't work.
Regards.