The default of vim to read .f90 is fixed format, and vim will displays a yellow color when you am beyond 72 column. Then you can change it by use these command
:let fortran_free_source=1
or add the line below in ~/.vimrc
let fortran_free_source=1
from Re: VIM : used for Fortran files .f or .f90
you can also see
:help fortran.vim