Hello,
You can use a view override to add your field in the form.
The idea is to use a field name "data[mail][param][...]" so the field value would be available within the email "user_order_notification" of HikaMarket.
Then, with an email override, you can add some content into the "preload" part to react to that parameter and add the vendor email as CC.
In the HikaMarket (and HikaShop) email you have access to the mailer object so you can interact with it as you want.
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.