Zencart does not have this functionality, and it is quite difficult to implement it.

Some people may have achieved the basis of what you are after by using the MULTI-SITE module. (see free add-ons section - link above).

Part of the challenge is monitoring orders by vendor. EG: if a customer orders 3 products - each from a different vendor - how does each vendor get paid? And how will the system segment the orders archive by vendor?

Lots of difficulties here...