Hi,
That is currently not possible the way you describe it in HikaMarket 1.x + HikaShop 2.x ; but it is planned to be possible in the future.
With HikaShop 3.x ; the checkout & cart system will be refactored and it will allow HikaMarket to have more feature for the cart management (and also the checkout).
Thanks to that, HikaMarket would be able to refuse the addition of a product into the cart and add it in another cart instead (more easily than with the current cart system).
It will also allow HikaMarket to have specific checkout workflow per vendor.
So with current version of HikaShop/HikaMarket ; the best is to use an adaptive payment method and the "shipping per vendor".
The customer will be able to choose one shipping method per vendor and complete his checkout.
Thanks to the adaptive payment method, all vendors will be pay in the same time (with one single transaction).
Otherwise, you have to activate the vendor limitation in the cart ; but when a customer will try to add a product from another vendor, it will just refuse the addition and it won't place the product in another cart. It could be possible to modify HikaMarket to do so but with the arrival of HikaShop 3, I think the best is to way a little bit.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.