You should habe a Look at Barbara Levick, Vespasian, London 1999 (2005 as paperback). It is a bit dry, but hast lots of Information and is more in the style of a biography, if that is what you're looking for.

I'm not sure if you're interested in historical fiction but there is a books series by Robert Fabbri that follows Vespasian from youth onward, as far as I know the major events are historically accurate but obviously the dialogue and drama are fiction. The first book is called Tribune of Rome.