What would Namor care about Vibranium? He attacks the surface world because it is threatening the oceans and his territory. That has everything to do with Wakanda being landlocked, because he has absolutely no reason to attack them, and they couldn't exactly pollute the oceans with Vibranium either. They aren't even near an ocean at all. They are locked in on every side by land. Namor wouldn't even be able to command the water to drown Wakanda because where's that water going to come from? If he's going to attack somewhere, it will be places like Angola, Gabon, Namibia, South Africa, Mozambique, Tanzania, Kenya or Somalia. And I'm only going by the bottom half of Africa at the moment as there are plenty of other places surrounded by ocean on the top half. The reason he attacked the US, particularly New York, was because that is on a coastline. He would never attack somewhere like New Mexico, Nebraska or Kansas. It would be like attacking the Bahamas because they're causing snow avalanches.