Create child product by code

[granduca]

I need to write a PHP code for creating a child product in Virtuemart 2.0 and Joomla 2.5.

Starting with some information of his father, at each execution some values ​​of the child must be customized (name, sku, quantity in stock, short description), including a new price

The code must be run externally to virtuemart.
Should I create a service, better if json, which may be invoked by a module of the site.

I think the code should include log in into Joomla / VM.

Where can I find information, sample code or guidelines for this operation?

thanks