Why "sudo nano" can bypass csrutil / SIP when enabled and other apps can't?
SIP doesn’t matter so much on Catalina. The system volume is read only / protected.
When you say crsutil was enabled, that is meaningless. That tool is always enabled and can run. You would need to report the full status it reports and then reboot the Mac and then specify the full path to a specific file you attempted to modify for someone to be able to check your work.
I would just set aside all the history and ask a follow on question showing your precise vim failure. That’s quick for people to answer whereas trying to teach anyone how SIP works takes about 2 hours and constant feedback to cover all the nuances. Far too broad for a question here other than the idea to ask a follow up question that documents the specifics.