vim call a function from inside a vmap - vim

Question

I have created a vmap text object for selecting the text of a single LaTeX \item:
vmap im ?\\item<CR>o/\\item\\|\\end{itemize}<CR>b$
But this has the annoying feature that I lose my current search term. I have read that search terms are restored when the search happens inside a function call, so I wanted to convert the map to just call a function that would do the searches:
function! ItemizeTextObject()
?\\item
normal o
/\\item|\\end{itemize}
normal b$
endfunction
vmap in :call ItemizeTextObject()<CR>
Unfortunately, this does not work: I get an error ("Pattern not found: \item|\end{itemize}"), no text at all is selected, and a new line is inserted below the line my cursor is on. I tried several variations of this, and had no success.
I think the basic problem is that I have to preserve the visual mode when calling the function (the o in my command should switch to the other end of the selection, but it inserts a new line instead), but I don't know how.
Update:
I try to get the following behaviour: In a text like this:
\begin{itemize}
\item lorem ipsum...
\item piece of text I want to select,
the *CURSOR* is here, and there is more text
that follows
\item lorem ipsum...
\end{itemize}
I want to hit vin, and then the text block in the middle should be selected:
\item piece of text I want to select,
the *CURSOR* is here, and there is more text
that follows
that means the text from the beginning of the previous \item, until but not including the next \item or \end{itemize}.

Answer 1

I've used the doc on operatorfunc to come up with the following, which should be (close to) what you want1:
function! ItemizeTextObject(type, ...)
let sel_save = &selection
let &selection = "inclusive"
let reg_save = ##
if a:0 " Invoked from Visual mode, use '< and '> marks.
silent! 1,+1?\\item
norm v | " use V for linewise visual mode
"" use V for linewise visual mode:
"norm V
silent! /\\item\|\\end{itemize}
"elseif a:type == 'line'
"elseif a:type == 'block'
else
silent! 1,+1?\\item
norm v
silent! /\\item
endif
norm b$
let &selection = sel_save
let ## = reg_save
endfunction
silent! unmap in
xnoremap <silent> in :<C-U>call ItemizeTextObject(visualmode(), 1)<CR>
If you want the mapping in both visual and select modes, you should use vnoremap
Notes of things to address:
you can now implement the motion from another mode (fill in the branches in the function)
if wrapscan is on, no search should wrap (perhaps temporarily set nowrapscan?)
you might want to make the operator repeating so you can extend the selection by saying vininin (see https://stackoverflow.com/a/7292271/85371 for an example)
it looks like you want 'linewise' behaviour (due to b$?)
consider using norm V (see comment)
Edit I compared the behaviour with this simple mapping:
xnoremap <silent>in ?\\item<CR>o/\\item\\|\\end{itemize}<CR>b$
1 Disclaimer: I don't know LateX