I'm want to tell you how happy i am and i really appreciate all your work guys. But...
I just updated virtuemart from 2.0.18b to 2.0.20b hoping that all or at least some of the stockable variant problems where resolved. I have updated the AIO also. In 2.0.18 i manage to reach the conclusion that the plugin VM Custom Field with Stock Control is acting strange, some times adds a child, sometimes is not, depending on variables that i cannot imagine. I have tried all the option described in this post but i wasn't able to present to the client a proof of working stockabe.
Well, after update to 2.0.20b the problem is sort of solved: The plugin is not working at all now. I press "new", i get some lines with childs products, i'm unhappy about the options that i get there (once i have pressed "new" i can't change the stock, only the attribute and price of the child) but the problem (excuse the irony) is resolved promptly once i press "save": The childs are never saved!
Looks like i'm stuck to the NOTstockable variants in this shop.