Send automated messages to vendors

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