Hello,
Either way, you need to store "somewhere" which user is "following" which vendor.
It's not something that HikaShop or HikaMarket will provide.
If you want to use Acymailing it would require to create a lot of groups for newsletters and maybe some new plugin to create the dynamic vendor filter.
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.