I would have thought that the primary vendor is almost always the one that is contacted as they are the ones that submitted the product to the MP.

When i partner up with someone, i always put myself as the first contact in the readme - if i am the one submitting the product that is.

I normally give a vendor 24 hrs to get back to me, if they dont and they have a partner, i then send the same sitemail to the partner and hope for the best.