Unless you are running a site where a single vendor's product is probably the only one (or multiples) in your cart (like a vacation rental site, etc.), why would you ever want to force a customer to go through a separate checkout process for every vendor who has items in their cart? And if you are running a vacation rental (or similar) site then your customers would rarely ever see multiple checkouts anyway. None of the major marketplace players implement like this (Amazon, etc) and from a customer perspective, I'd leave after seeing my first checkout didn't include all that I ordered (having a separate shipping section is fine - separate payment methods should be optional).
If you have a different perspective on this, I'd really like to hear the advantage of having a customer do multiple checkouts for products they've added to their cart.
What's needed (from my customer feedback and development requests) is:
1) Category commissions (if you want this in standard MVE, you can use our EZ Vendor Plans Plus addon).
2) Vendor promotions on their products (catalog promotions including free shipping).
3) Configurable signup data including legal documents like TOS, licensing, tax and other business legal agreements, etc.
4) Implement the "suppliers" addon (multiple warehouses) within the context of a vendor.
5) Ability to assign tax to vendor or merchant for payment (right now, it's part of the commissioned amount which is just wrong).