You say this is obvious, but it is not. In fact there is no proof that this ever happened. In fact you simply assume it happened. You don't think you have to prove it, because it's just so obvious. That's not science.
Natural selection is not the same thing as evolution. Natural selection is animals adapting to their environment. The theory of evolution goes much further and says that all animals are descended from one common ancestor. If you don't know the difference between natural selection and speciation, then you are in fact completely unlearned in the the field of biology, unlike me.
If the non-vulture became a vulture over many generations, (something you have no proof of) then it would at some point have stopped being a non-vulture, and become a vulture. All you have to do is prove that a non-vulture gave birth to a vulture at some time, but you won't ever be able to do that, because non-vultures don't give birth to vultures, they only give birth to non-vultures.
How?
What was the mechanism of this adaptation?
If they look the same (i.e. ‘like a vulture’) because they “adapted the same way to the same environment” - then it follows logically that previously they did not look the same (i.e. ‘like a vulture’).
So if we are to accept YOUR reasoning that they look the same because they “adapted the same way to the same environment” - then two different bird populations adapted the same way to the same environment through adaptations that MADE THEM LOOK LIKE VULTURES.
And once again, speciation is not a case of a non vulture giving birth to a vulture in one generation (as only one unlearned in biology would have it).
So what mechanism would cause these two populations to look the same through “adapted the same way to the same environment”? And what would you call them BEFORE they “adapted the same way to the same environment”?