If your goal is to create a cart and then have the user checkout, you might want to consider this approach instead.
It can be done with a pre-controller. No need to use the api at all. You can use straight html (I.e. an anchor tag) to do it all. See this for structure. You can add your own security related requirements if you need to.
Regarding orders - in Multi-Vendor, if customer places an order with products from multiple vendors, it gets split into multiple orders (one for each vendor), so each vendor will see only his part of order. Probably it works the same way for API requests - you have to create a separate order for each vendor that is included in order.