Insert tabs at start of line and spaces elsewhere

VaryTabs.vim

function! VaryTabs()
  " Description: Make <Tab> put tabs at start of line and spaces elsewhere
  if &expandtab
    return "\<Tab>"
  else
    let nonwhite = matchend(getline('.'),'\S')
    if nonwhite < 0 || col('.') <= nonwhite
      return "\<Tab>"
    else
      let pos = virtcol('.')
      let num = pos + &tabstop
      let num = num - (num % &tabstop) - pos +1
      return repeat(" ",num)
    endif
  endif
endfunction
inoremap <Tab> <C-R>=VaryTabs()<CR>