Answer by thdoan for Turning off auto indent when pasting text into vim

While setting the paste mode with paste/nopaste/pastetoggle is perfectly fine, you still have to manually enable paste mode before pasting and disable paste mode after pasting. Being the lazy person that I am, below is the best solution that I've found so far, which automatically toggles the paste mode when you paste.

Here's a little trick that uses terminal's bracketed paste mode to automatically set/unset Vim's paste mode when you paste. Put following in your .vimrc:

let &t_SI .= "\<Esc>[?2004h"
let &t_EI .= "\<Esc>[?2004l"

inoremap <special> <expr> <Esc>[200~ XTermPasteBegin()

function! XTermPasteBegin()
  set pastetoggle=<Esc>[201~
  set paste
  return ""
endfunction

Now you can paste without explicitly turning paste mode on/off - it is handled automatically for you.

Source: Coderwall

Note: This solution doesn't work in WSL (Windows 10 Subsystem for Linux). If anyone has a solution for WSL, please update this answer or add it in the comments.

Tmux If using tmux, then the declarations need to be double escaped. The code for this is also in Coderwall