:vsplit (short form: :vs) split the Vim viewport vertically. :30vs splits the viewport, making the new window 30 characters wide. Once
:vsplit
:vs
:30vs
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 |