vte_terminal_fork_command_full() is unusable with a custom child setup function, since there's no way to pass the VtePty to it, but that's required since calling vte_pty_child_setup() is mandatory in custom child setup functions.

For 1.0 we can just break API, but until then another solution is needed. Perhaps use one of the free bits in GSpawnFlags for a vte flag that makes __vte_pty_spawn() put the VtePty* into the passed child_setup_data at offset 0 ?