How do I remove first 5 characters in each line in a text file using vi?

As all lines are lined up, you don't need a substitution to solve this problem. Just bring the cursor to the top left position (gg), then: CTRL+vGwlx

:%s/^.\{0,5\}// should do the trick. It also handles cases where there are less than 5 characters.

Use the regular expression ^..... to match the first 5 characters of each line. use it in a global substitution:

:%s/^.....//