Hi,
Such kind of customization will be possible thanks to the next release.
With HikaMarket 1.3.1, you will be able to set a cart vendor limitation. Thanks to it, you will be sure that only products from one vendors are in the cart.
Otherwise, how could you know if you have to modify the checkout workflow if there could have products from other vendors in the cart ?!
So, with this new feature, we can imagine such feature.
Like that I can't give you precise information on the best method to override the checkout workflow but what I am is, without the cart vendor limitation, it is not something that we can imagine.
Regards,