There are a number of email marketing plugins available. I should think who has the ability to send out emails are managed by role and capabilities, but the default mechanism likely would convey greater capability than would be appropriate for vendors. Some customizing may be needed. These plugins might also not segregate newsletters by specific users. You may need the help of a membership type of plugin to better manage who has access to what. Some custom coding might be necessary to properly integrate the two plugins.
If I may make an unsolicited observation, unless mass email marketing is properly handled, it’s a very good way to get your server and domain name blacklisted as a source of spam. Unless you trust your vendors to properly manage marketing campaigns, your desire might be a bad idea. You might be better off letting vendors manage email marketing through any of the many online services intended for the purpose.
Even if you don’t care about the blacklisting possibility, it’s important that emails are properly composed to have any chance of the email actually being delivered and not being discarded as spam. Messages must have proper DKIM, DMARC, and SPF headers. For that to happen, an email’s From: address usually needs to belong to the server’s domain name. While it’s possible to send email with From: addresses like [email protected], such messages are more likely to be treated as spam and never be delivered.
