The mapping you defined works in normal, visual and operator-pending modes and does not automatically change modes. That is, when you type
h, you effectively send to Vim the key sequence
vert h, which enters visual mode (
v), selects to the end of the current word (
e), and replaces the visually-selected region (
r) with the character
t. Then, back in normal mode, the cursor is moved one position to the right (
) and one position to the left (
The mapping you want is this:
:nnoremap h :vert h<CR>
The colon (
:) is a necessary part of the
:vert command, putting Vim into ex mode before accepting the command. The
<CR> terminates the command as you would if you had typed it.
:nnoremap rather than
:noremap because it didn't seem useful to have this mapping work in any but the normal mode.