Vim can't scroll in iTerm2

Recent versions of vim support the "SGR" protocol, you can add code bellow to your .vimrc:

set mouse=a
if has("mouse_sgr")
    set ttymouse=sgr
else
    set ttymouse=xterm2
end

I had the same issue, and Valery's answer did not work for me. What did work is checking the "Enable mouse reporting" box under Profiles > Terminal in the iTerm preferences.