Installed Marketpress and then CustomPress. Created a custom field for Vendor for post type product. Id of the field is ct_Vendor_text_866b
This field is displayed in the add product screen in MarketPress and I can use a CSV plugin to import the data and the vendor data is displayed in the Marketpress product admin screen. So far so good.
I then used the wordpress xmlrpc api http://codex.wordpress.org/XML-RPC_WordPress_API/Posts#wp.newPost
to create a new product. For the custom fields section, I added mp_price and ct_Vendor_text_866b to the array.
The api call returns successfully, a new product is added, the title and content fields are set correctly. The price is also populated correctly. Only the field (Vendor) created with CustomPress is not populated.