I have a shop where I'm selling clothes. You can off course choose different sizes and I have managed to set up child products with stockable variants. But on the product detail page, the default size is the first item in the list. That means if a user forgets to choose size, he will by default order size SMALL. That is not good. I want the default choice to be "Choose size" or something like that. And if the user forgets to choose size, he should be prompt to make a choice.
So, is that possible with Virtuemart?
I do not know the answer to your question but I wanted to ask you if you have the variants working? My options show on the page, but you can't add them to cart because VM thinks they are out of stock.
For your problem, You could always make Choose size the first option then you know when they forget vs purposely order a small.