This seems to be a known issue, see the comment in ftplugin/latex-suite/envmacros.vim:
" Only include the <M-i> mapping if the user want this. Note that it
" conflicts with inserting 'é'.
if !hasmapto("<Plug>Tex_InsertItemOnThisLine", "i") && g:Tex_AdvancedMath == 1
imap <buffer> <M-i> <Plug>Tex_InsertItemOnThisLine
From the vim-latex help
By default, the mappings involving the key are turned off for compatibility with inserting non-ASCII characters. It can be enabled by setting
let g:Tex_AdvancedMath = 1
Curiously, I couldn't find this in the online documentation about Alt Key Macros which starts at:
By default, typing
Alt-<key> in Vim takes focus to the menu bar if a menu with the hotkey
<key> exists. If in your case, there are conflicts due to this behavior, you will need to set
$VIM/ftplugin/tex.vim in order to use these maps.
:map-alt-keys for more information on how Vim handles