László Vadnay was a professional soldier. He started his military service in 1917 and competed in shooting for HOTIS in Debrecen. In 1935 and 1936, he won the individual championship in the automatic sports pistol competition. In 1938, he also won a team championship in the small-bore sport pistol competition, now in the colors of Buda HOTIS.
Between 1935 and 1942, Vadnay appeared eight times in the Hungarian national team. In 1936, he participated at the Berlin Olympics where he finished 10th in the rapid-fire pistol competition. In 1939, he took first place in the automatic sports pistol team competition and fourth place in the sports pistol team competition at the World Championships. At the 1937 World Championship he finished sixth in the sports pistol team competition. From 1940 to 1944, he was also the head coach of the Hungarian shooting team.
He left Hungary in 1945 and lived first in Brazil and then in the United States.