Is Vịnh Mexico more common than Vịnh México? If not I would propose:
name:vi=Vịnh México
alt_name:vi=Vịnh Mễ Tây Cơ;Vịnh Mễ;Vịnh Mexico;Vịnh Mê-hi-cô

In case you agree, could you do me a favor and change alt_name:he to name:he as well? I tried to encourage the mapper to do that and restore alt_name:vi themself in a private message. This learning opportunity for a somewhat experienced mapper meant that you changed it ahead of me. I hope that didn’t bother you and I will change alt_name:he myself if I am mistaken in my assumption that Vịnh México is more common.