To get the secret token in Vietnam, you need to be either the D-5 Hondo or the Baja Truck. It also helps to turn on the infinite counter-measure code [at the pause screen, press Y(3), X(2), Z(2)] You need to go to the open ocean part and find the volcano island that you seemingly can't get to. Go out into the water, and when you start to sink, use your Vertical Boost counter-measure. Keep using this and heading towards the island until you make it. The token is on the island.