These documents are relevant to the question of what von Neumann knew of Turing's work on computability before the Second World War, and so also to the question of how von Neumann could have drawn on Turing's ideas when formulating the EDVAC report in 1945.
The first is von Neumann's formal letter of reference dated June 1, 1937, supporting Turing's application for a Procter Fellowship at Princeton for the year 1937-38. The application was successful. The point of interest is that von Neumann made no reference at all to Turing's On computable numbers, which had been published some six months earlier. Source: Princeton University.The second is a letter to me (Andrew Hodges) from von Neumann's colleague S. Ulam, postmarked 18 April 1979. This indicates that by 1939 von Neumann was aware of Turing machines. It is worthwhile noting, however, that Ulam's interest was, like von Neumann's, focussed on Turing's work in continuous groups.
The letter that Ulam sent to Turing in 1939 does not seem to have survived. A letter from von Neumann to Turing can be seen in the Turing Digital Archive here. Again, the subject matter is that of continuous groups.
Both documents were quoted in chapter 3 of Alan Turing: the enigma.