I have a 91'civic it cranks fine when it's cold but after it runs for 15 or 20 minutes. It shuts off what can cause this to happen?

Replace the Ignition Control Module (ICM) and coil:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post265896

Replace the Ignition Control Module (ICM) and coil:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post265896

Could vapor lock cause this

Nope.

Maybe I should replace the ICM, coil, distributor housing, main relay, and ECM, or just take it to the honda dealership

If the CEL comes on and goes off normally, when the ignition switch is turned to ON (Position II), the main relay and ECM are probably fine. I would start by replacing the ICM and coil. These are very problematic components on Hondas with over 120,000 miles, or 10 years, on them.