Dealing with tabs and spacing in Vim

.vimrc

" The final answer to the Tab Question
set colorcolumn=80
set shiftwidth=4
set tabstop=4
set softtabstop=4
set expandtab
set list listchars=tab:»·,trail:·

function! SwitchTab()
    setlocal expandtab!
endfunction
vnoremap <Leader><tab> <Esc>:call SwitchTab()<CR>
inoremap <Leader><tab> <Esc>:call SwitchTab()<CR>
nnoremap <Leader><tab> <Esc>:call SwitchTab()<CR>

augroup wspace
    autocmd!
    " Visual warning about trailing whitespace and mixed up spacing
    highlight ExtraWhitespace ctermbg=red guibg=red
    match ExtraWhitespace /\s\+$\| \+\t\|\t \+/
    autocmd BufWinEnter * match ExtraWhitespace /\s\+$\| \+\t\|\t \+/
    autocmd InsertEnter * match ExtraWhitespace /\s\+$\| \+\t\|\t \+/
    autocmd InsertLeave * match ExtraWhitespace /\s\+$\| \+\t\|\t \+/
    autocmd BufWinLeave * call clearmatches()
augroup END