Try:

:vert sb N

which will open a left vertical split (by default, unless you have modified some options).

To open a split to the right, on the other hand:

:vert belowright sb N

You can use :split and :vsplit to divide the current area into two windows with the same buffer. If you supply an argument then one of the new windows is created with the argument as a file name used for the buffer in the new window

To gain more control over how the splits are created, you can combine the :vertical, :leftabove and :rightbelow commands. Also of use are the the :sfind and :sb commands.

Examples

:vertical sb 3

Create a vertical split and show buffer number 3 in the window to the left.

:vertical rightbelow sfind file.txt

Create a vertical split and read file.txt into the buffer in the right window.

:rightbelow sfind file.txt

Create a horizontal split and read file.txt into the buffer in the bottom window.

Navigating splitsEdit

Use Ctrl-W followed by one of the hjkl movement keys.