Dear Jansy,
I don't know if it is necessary to wonder where VN got his impressions of "Et in Arcadia Ego." In Letter 248 (August 7, 1957) of the VN-Wilson letters, VN writes, "My source for understanding et in Arcadia ego, meaning 'I (Death) (exist) even in Arcady,' is an excellent essay in Erwin Panofsky's The Meaning of the Visual Arts, Anchor Books, New York, 1955."
Panofsky's book remains widely available.
Best,
Matt