How can I automatically message/email all vendors periodically eg each month in Multi-vendor?
You can use the Newsletters module. Just create new mailing list and add all vendor e-mail to it. Then create newsletter and send it to the new mailing list subscribers
How can I email them automatically? I want to remind vendors to update their stock and prices each month
Also in Newsletters am not able to search for Vendors - it shows up blank when trying to add users. And Newlsetter add-on only lets me add users, not a list like CSV
Out of the box you can send it only manually. Use integrations with e-mail services to send it automatically (like Unisender)
Yes, but you can add e-mail manually