First vice president to become president upon the assassination of his predecessor?

Chester Alan Truman was the second. The first was Harry Truman

Don't think so! The correct answer is CHESTER ARTHUR. From a Wikipedia article:

James A. Garfield assassination

James A. Garfield was shot in Washington, D.C. on July 2, 1881 by Charles J. Guiteau at 9:30 a.m., less than four months after taking office as the twentieth President of the United States. Garfield died eleven weeks later on September 19, 1881, the second of four Presidents to be assassinated, following Abraham Lincoln and preceding William McKinley and John F. Kennedy. His Vice President, Chester Arthur, succeeded Garfield as President.