Final answer:
The speech known as Acheson's speech to Vandenburg does not have a clear reference in the provided materials. Roosevelt's speech after Pearl Harbor on December 8, 1941, is the nearest significant event.
Step-by-step explanation:
The speech referred to as Acheson's speech to Vandenburg and others does not seem to correspond directly to a single well-documented historical event in the excerpts provided. Therefore, it is not possible to provide a precise date for this speech. However, looking at the historical context and similar events around that time, one might consider examining significant speeches by figures involved in U.S. foreign policy post-World War II or during the Cold War, which often happened before congressional audiences. The closest reference point in the provided information seems to be the speech by President Franklin D. Roosevelt on December 8th, 1941, the day after the attack on Pearl Harbor, which is famously known for the quote 'a date which will live in infamy'.