Use Alt-Mouse to select blockwise
From Vim Tips Wiki
created 2004 · complexity basic · author hari_vim · version 6.0
If you use mouse to make visual selections, you might have observed that making blockwise selections is cumbersome (you need to click <LeftMouse> precisely four times). To make it easier, you can make Vim behave like MS Word using the Alt modifier.
Place the following lines in your vimrc:
noremap <M-LeftMouse> <LeftMouse><Esc><C-V> noremap <M-LeftDrag> <LeftDrag>
You can also change the modifiers and the mousekey to anything else that is valid, e.g:
noremap <S-RightMouse> <LeftMouse><Esc><C-V> noremap <S-RightDrag> <LeftDrag>
One does not need to do four-clicks to get a rectangular selection. Simply click and hold in the upper left location. Drag to the lower right location and release (If on another page release the mouse and then right click the lower right location). Type control-v you now have the rectangle selection. (at this point the right click can change the corner points of your selection). With this selection, you can Yank, delete, upper-case, convert case, or any other editing command such as substitution. Of couse, I'm using gvim and not just vim.
Remember that under Windows, by default gvim is configured to use Control-V for Paste, because that's the standard way most Windows apps work, so in Windows one must drag through the region and then use Control-Q, because the usual Vim purposes of Control-V are remapped onto Control-Q.
Here is a different version from Gerald Lai. This one automatically creates the selection from current cursor position to the mouse click.
nnoremap <A-S-LeftMouse> mz<LeftMouse><C-v>g`zo<C-g> inoremap <A-S-LeftMouse> <Esc><C-v>g`^mz<Esc>gi<LeftMouse><C-o><C-v>g`zo<C-g> vnoremap <A-S-LeftDrag> <LeftDrag> vnoremap <A-S-LeftMouse> <C-v><LeftMouse>mzgvg`z<C-g>