You are correct in your assessment that you'll need to send a notification and not a reminder in order to send it as an 'Open' email.
Notifications are triggered when a record is added, modified, or deleted.
In your case, you'll need to set up a trigger when the record is modified. This is only achieved when the record is actually edited and not simply updated as part of a formula.
Automations allow you to modify records automatically (ie without manually changing the record or pressing an API type button). Since you have over 1000 vendors automation is the way to go.
My recommendation would be:
- Create a new formula checkbox to 'check' when Today()= the date you want the notification to send.
- Create a text field where you'll indicate that the notification was 'sent'
- Set up an automation (located in the settings for your app) on every day to trigger when the checkbox you created is checked
- Then modify the new text field with value 'sent'
This should initiate the notification to send once per day to all of the vendors who should get it. I have not tested it.
