Hi. I'm Using VM 2.0.8a.
There is a bug, don't know if that's new with VM 2.0.8a, but the custom field with plugin "VM-CUSTOM, stockable variants" won't work.
Tested on my test website, and into a fresh new install.
Description of the problem :When you add a product variant (child) to your shopping cart, the SKU used is not the good one !
The SKU's are replaced by the last item's SKU that has been added as child product.

The size number 10 has the SKU NUMBER: SKU-for-Size-10 and the size 12 has the SKU NUMBER: SKU-for-Size-12 (the rests have blank SKU which will replaced by parent product's SKU. Okay? And I added the 8, 10 and 12 sizes to cart and look at the screenshots and see how it looks like:

So actually, there is no working solution to have produt stockable variants (with SKU) :/