He did his Ph.D. at Duke under the supervision of Richard B. Hays. His dissertation was published by Brill as part of Supplements to Novum Testamentum in 2002 under the title Heralds of the Good News: Isaiah and Paul "In Concert" in the Letter to the Romans. It's an excellent study of Paul's quotations of and allusions to Isaiah in Romans 9-11, utilizing Hays's theory of intertextuality set forth in his groundbreaking and influential work Echoes of Scriptures in the Letters of Paul. Here is a good summary of Wagner's thesis in his own words: