Pad all lines with spaces to a fixed width in Vim or using sed, awk, etc

Vim

:%s/.*/\=printf('%-63s', submatch(0))

$ awk '{printf "%-63s\n", $0}' testfile > newfile

In Vim, I would use the following command:

:%s/$/\=repeat(' ',64-virtcol('$'))

(The use of the virtcol() function, as opposed to the col() one, is guided by the necessity to properly handle tab characters as well as multibyte non-ASCII characters that might occur in the text.)