While you are trying to open alert_SID.log file or some other large files with vi editor you may hit ex: 0602-101 Out of memory saving lines for undo  error message,

# cd /u01/app/oracle/admin/DB_NAME/bdump
# vi alert_SID.log
ex: 0602-101 Out of memory saving lines for undo.

So you can use below syntax to can set EXINIT value,then you can retry to open your alert.log  file.

export EXINIT=”set ll=80000000″

Reference:
Hints and tips when using vi on AIX