Set the Instant Pot to the sauté setting and let it get hot. Add the butter, and when melted, add the onion, celery, and bell pepper and cook, stirring occasionally until soft (about 4-5 minutes). Add garlic and cook for an additional minute.
Add the peas, chicken broth, ham hock, salt, pepper, and cayenne. Switch the Instant Pot to manual, add the lid and check that the value is correctly positioned for sealing. Set time for 20 minutes on Manual mode. (Note--with this much stuff in the IP, it will take about 20 minutes to come to pressure)
When the timer is up and the IP beeps, let the pressure release naturally for 20 minutes, then do a quick pressure release if the pin hasn't dropped already. Remove IP lid and stir in vinegar. Check for seasonings and enjoy!