Hi,
1. I think it would be possible to use the "mass action" system in order to assign several products to a vendor ; there is no special tool in HikaMarket for such thing but it's a good idea.
2. You can assign the main product with the vendor, the variants will be automatically assign to that same vendor (But you can override it by setting a specific vendor for a specific variant).
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.