How to increase the vertical split window size in Vim

:vsplit (short form: :vs) split the Vim viewport vertically. :30vs splits the viewport, making the new window 30 characters wide. Once this 30 char window is created, how would one change it's size to 31 or 29?

With horizontal windows Ctrl-W + increases the number of lines by one. What is the equivalent command to increase the columns by one?

CTRL-W >

and

CTRL-W <

to make the window wider or narrower.

And Ctr-W =

will make them equal

In case you need HORIZONTAL SPLIT resize as well:
The command is the same for all splits, just the parameter changes:

- + instead of < >

Examples:
Decrease horizontal size by 10 columns

:10winc -

Increase horizontal size by 30 columns

:30winc +

or within normal mode:

Horizontal splits

10 CTRL+w -

30 CTRL+w +

Vertical splits

10 CTRL+w < (decrease)

30 CTRL+w > (increase)

Another tip from my side:

In order to set the window's width to let's say exactly 80 columns, use

80 CTRL+W |

In order to set it to maximum width, just omit the preceding number:

CTRL+W |

I have these mapped in my .gvimrc to let me hit command-[arrow] to move the height and width of my current window around:

" resize current buffer by +/- 5 
nnoremap <D-left> :vertical resize -5<cr>
nnoremap <D-down> :resize +5<cr>
nnoremap <D-up> :resize -5<cr>
nnoremap <D-right> :vertical resize +5<cr>

For MacVim, you have to put them in your .gvimrc (and not your .vimrc) as they'll otherwise get overwritten by the system .gvimrc