What conditions do I need to generate an email as soon as a vendor payment has been created? 'Create' (on the workflow') and 'Create' on the state event type seem insufficient.

If you can live with a 30 minute delay you can do a very simple workflow with a checkbox that defaults checked and have your workflow uncheck the box and send the email.

Here's what I tried. Created a new check box, and a saved search to find records which the check box ticked. The search powers the work flow, and the state actions (scheduled trigger) sends the email and unchecks the boxes. No beer 😕

you've probably done something small incorrect.