Hi,
If you want your vendor be paid directly (using a payment method using a credit card), the limitation of vendors in the cart is some kind of obligation.
Otherwise you can use "adaptive" payment plugins or manual payment methods ("collect on delivery").
There is no best practice.
When a customer already have a vendor product in his cart, he cannot add products of other vendors. The product addition will be cancel and he will get a notice message.
In order to avoid the customer to perform unwanted action, you can organize your shop by vendors ; so when the customer enter a vendor section, he will know that he can buy all the products from the same vendor.
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.