Hi,
If you have the "Add partner to user account during registration" setting activated in the HikaShop configuration, then the affiiate will be assigned to the client automatically during the registration provided that the client clicked on a link of that affiliate. Then, the orders placed by that client would automatically get that affiliate assigned to them based on that.
However, there is no option to see that assignation or change it in the backend interface.
What you could do is use the "display the results" action of a mass action on the users via the menu System>Mass actions on the columns "user_email" and "user_partner_id" so that you could double click on the "user_partner_id" cell of the result table to modify the value in that column for that client to the id of the affiliate you want.
HikaMarket can have commissions, but you can't attach vendors to clients. Vendors are attached to products, or you can attached them to zones (based on the address of the client) or let the client select the vendor during the checkout.
I'm afraid that none of these scenarios would help you.
There is no automatic system to allow affiliates to use their commissions on the shop. However, you can mark their commissions as paid manually, and after that, you can either create a coupon of the same value and give them the coupon code, or if you setup the user points system, you can add the corresponding points to their user account.