In the Marketplace where a product has more than one vendor listed I suggest that one of the vendors is nominated as the primary vendor to be contacted for product support.  This would ensure that a customers concerns would get addressed by the vendor most familiar with and has the most knowledge of the product.  

This could easily be addressed at the time of uploading the product for testing.  An additional field requesting the nominated vendor for product support could be added.  This would remove the need to delete Marketplace items that have more than one vendor listed simply because one of the vendors does not log in frequently enough to provide this service.

Or at the very least,  have the vendors include in their discription which one should be contacted for different types of questions.

Example;  if one vendor did the modling,  and the other did all the textureing,  include that in the discription, so I know to contact the one who did the texturing if I have a question about the textures.

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.