On the visit page, there is a pencil icon next to built-in forms to "Edit" them. Custom forms need that capability too. If the custom form is configured to use the Standard UI, it should be edited with the Standard UI. If it is configured to use the Simple UI, it should be edited with the Simple UI.
Also, while you're in there... the Vitals form is created with the SimpleUi, but edited with the StandardUi. I would expect them to both use the same; Simple Ui. (RA-457)