Use "terminal" input purpose
Gtk (3 and 4) now supports GTK_INPUT_PURPOSE_TERMINAL
(though only meaningful on Wayland at present): gtk!1245 (merged)
Unfortunately this missed the 3.24.13 release but will be in 3.24.14, clearly this is something we want Vte to use
The idea here is that an OSK can show things like Ctrl/Arrow/Fn keys that aren't relevant to your average text field, this is especially important with all the "linux phone" projects at the moment