For freebsd 9 (starting with 9.2 I think) and later (namely since the import of netbsd's vis(1) implementation in 9-current), vis -F doesn't fold.
9-stable% echo 12345678 | vis -F 5
8-stable% echo 12345678 | vis -F 5
I haven't checked netbsd yet. Please add a note here if you do.