So how does his "vested interest" affect the veracity of his claims?

I have an observation to share with you: If we ignore everyone who has some sort of "vested interest" in a subject, the only people we can consult for advice or suggestions will be people who are completely ignorant of the necessary facts and who have no experience in the subject matter.

Expertise in a subject is gained through working in the field. Working in the field means acquiring some sort of personal interest in the outcome.

I find that bias is useful to consider when given unfounded claims. For example, if someone says "I think Facebook is a complete waste of time," you might ask whether they have a vested interest in making us believe this is the case. However, if someone says, "Before Facebook there was MySpace and ICQ and Compuserve and Orkut and all of these foundered and died, Facebook is just the latest fad, it will die as well," we can analyze the claim directly and ask whether Facebook is a fad and if not, how it differs from the examples cited.