While the viral nature of the “phage” was only widely accepted in the 1940s, attempts to use the phenomenon in treating infections started early. After raising hopes in the interwar years, by 1945 phage therapy had been abandoned almost entirely in the West, until the recent revival of interest in response to the crisis of antibiotic resistance.
Is phage therapy at all relevant or applicable in vaccine development?
No, as far as I know phages target bacteria. So it’s rather an alternative to antibiotics.