Looking at the docstring for
C-h f visual-line-mode RET):
(visual-line-mode &optional ARG)
Toggle visual line based editing (Visual Line mode) in the current buffer.
If called from Lisp, toggle
the mode if ARG is ‘toggle’, disable the mode if ARG is
a non-positive integer, and enable the mode otherwise
(including if ARG is omitted or nil or a positive integer).
so you have to provide a non positive integer argument.
Note this is the recommended behavior for minor mode commands, as the elisp manual states:
The mode command should accept one optional argument. If called interactively with no prefix argument, it should toggle the mode (i.e., enable if it is disabled, and disable if it is enabled). If called interactively with a prefix argument, it should enable the mode if the argument is positive and disable it otherwise.
If the mode command is called from Lisp (i.e., non-interactively), it should enable the mode if the argument is omitted or nil; it should toggle the mode if the argument is the symbol toggle; otherwise it should treat the argument in the same way as for an interactive call with a numeric prefix argument, as described above.