Hello,
The HikaShop product contact are sent to the vendor when you have HikaMarket ; thus I don't understand your message regarding that the administrator need to answer to all queries.
PMS are Private message System, as previously explained, these are message between two Joomla accounts. You can use a view override in the HikaMarket vendor page to add a link to your PMS so the customer can send a PM to the vendor admin account.
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.