Hello,
The "our of stock" HikaShop email cannot be override by HikaMarket since it can includes multiple products (of different vendors) and the links are backend urls.
In order to design a plugin which will notify vendors of their product stocks, it would requires full specifications and take in consideration a lot of different usages. You should not notify several times a vendor for a product without stock, it could "spam" the vendor for something which is "normal".
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.