At the age of 16, Vassilev received his first important portrait commission to paint Bulgaria's Secretary of War, General Lazaroff.
In 1929, he completed his masterpiece portrait of the Archbishop Angelo Roncalli, then the papal nuncio to Bulgaria.
He also established a studio in West Palm Beach, where he continued to paint political officials and socialites such as Helen Rich.
[2] His friendships included Ernest Hemingway, President Harry S. Truman and Jack Dempsey.
In 1989, Norton Gallery of Art in West Palm Beach, Florida, held a retrospective of his work.