Can I use vi to edit main.cf?

You can edit "main.cf" only when VCS is shutdown. VCS does not read main.cf when it is already running. VCS only reads from the configuration it has in memory. If you edit main.cf while VCS is running, VCS will not read your updates, and it will overwrite the main.cf with the configuration it has in memory. 

You can always edit a copy of main.cf and shutdown VCS, move the new main.cf into /etc/VRTSvcs/conf/config/ and restart VCS. Here's an example... 

1) haconf -dump 

2) cp main.cf /tmp/main.cf 

3) vi /tmp/main.cf 

4) haconf -dump -makero

5) hastop -all -force 

6) cp /tmp/main.cf /etc/VRTSvcs/conf/config/ 

7) hastart