On the other hand, the Vow did not change Harry's terminal goals.

If Coscott is right about the Vow protecting "the world" and not "its people", then it very much did change Harry's terminal values.

it seems reasonable to assume that the intended meaning of the speaker, rather than the actual words he used, is binding

The intended meaning of the three persons making the vow have to match, or the Vow won't work. And I think that two randomly chosen Death Eaters, who have absolutely no idea that people could survive without the Earth, who don't even suspect that there's been manned spaceflight, would indeed think that "the world" is "the Earth".