Can I send a Reminder Email to an Outside Vendor who's insurance is expiring?

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:

1. Create a new formula checkbox to 'check' when Today()= the date you want the notification to send.
2. Create a text field where you'll indicate that the notification was 'sent'
3. Set up an automation (located in the settings for your app) on every day to trigger when the checkbox you created is checked
4. 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.

I was wondering this myself if Automations could be used to trigger mass external emails.  I did a test and it seemed to work for me.

Actually, in a subsequent release, they changed these large updates to be done as a mass update so that method of triggering individual emails will no longer work.

Yet another solution is to use script. You create an extra field called [Dirty] and through script update it with either a random number of the current time. This is called "dirtying the record." It is easy to implement. The [Dirty] field does not carry any supplemental information and is only a mechanism to get the record to change and trigger notification.

I would argue that any script technique is superior to all other methods mentioned here not because of its individual performance but because it will open up your world to what can be accomplish with script. All the other solutions require you to do one-off setups and don't substantially expand your capabilities or portfolio of skills. I say script  everything!

We are attempting to accomplish something similar. Is script still the only way to accomplish this?

Yes. Contact me at QuickBaseCoach.com if you would like assistance