He shakes his head minutely. "They will see my failure to accept their invitation as a fault of my human heritage. They will not accept that a Vulcan turned away from their ways."

Certainly, he may identify himself as Vulcan but other Vulcans do not see him that way. To them, he is human. To humans, he is indistinguishable from other Vulcans. This is why he does not belong in either society.