That’s dated April 24.
It’s not, the administration by in large knew FDR was dying and would likely die before leaving office. Truman effectively was a president in waiting rather than a vice president.
Yes that he needed to speak about something important. Draw your own implication but it doesn’t actually say what your implying it says.
And if you wanted to know what Truman thought of not being told as vice president. (from the same link)
On June 17, Truman received a phone call from Stimson, who told him that the Pasco plant was “part of a very important secret development.” Fortunately, Stimson did not need to explain further: Truman, a veteran and a patriot, understood immediately that he was treading on dangerous ground. Before Stimson could continue, Truman assured the secretary “you won t have to say another word to me. Whenever you say that [something is highly secret] to me that’s all I want to hear. If [the plant] is for a specific purpose and you think it’s all right, that’s all I need to know.” Stimson replied that the purpose was not only secret, but “unique.”
