Okay, either me or the ref. may have been wrong. but I distinctly remember "Veert", because it was non-intuitive to me (as the way to pronounce Wirth), as a guy who didn't know any German at the time, only English. So the ref., probably.
It's not really wrong. There are English accents (such as Received Pronunciation) where an "ee" before an "r" is normally pronounced with an [ɪ] like in "wit". In any case, even if you pronounce the "ee" as something else like [i], "Veert" is probably still the sequence of letters that maximises the likelihood that an English speaker will understand by it something close to the true German pronunciation ([vɪʁt] or [vɪɐt]). "Virt", for example, would be read by most people as [vɜrt] (rhyming with "hurt") which to my ear is further off from the correct pronunciation compared to something like [viət].
"Veert" is correct, in the sense that it's how a German would pronounce it. Of course, the great man wasn't German; I don't know how he pronounced his own surname.
"Wit" is just wrong. Perhaps that was a joke that I missed about the man's humour.
He was Swiss, more exactly from the city of Winterthur located in the canton (state) of Zürich. The canton's official language is German, however. Of course, people over there speak in a strong local dialect called "Züritüütsch".
You just elongate the vowel i.e. pronounce it longer. The double „ü“ just indicates this vowel to be stressed. Dialects do not follow a strict orthography, however, so you might find it written slightly differently in other contexts.
Wirth lived in the United States for some time throughout his life but is a Zürich native. He must have spoken Züritüütsch („Zurich German“) privately, I am pretty sure (without having known him personally).
Umlauts aren't diphtongs; it's the same sound all the way through. GP used two consecutive ones in order to show that the sound is long. (And whaddoino, if the dialect has an official orthography, maybe that's how it's supposed to be spelled.)
