First guess will be to check ulimit settings (as root you can manually view the file /etc/security/limits). Check for differences between a failing user and a working user.

And please look at the coredump (using kdb). It has been a very long time since I have used adb (the first core debugger), so I cannot rattle off the commands needed to view what instruction caused the core dump. But there is lots of uesful information there. You can even run a program, and set break points, to find a possible cause.

segmentation fault means, btw, that an illegal address is being accessed.

Another idea: verify that working/failing users dont have a different LIBPATH setting. vi uses shared libraries, and if they wrong library is being used, you may get very strange behavior.