I don't really understand what I purchased outside of a new character class and a few new zones. All the monsters look the same (or very similar) to the ones in the base campaigns. The environments are nothing special and the dungeons use mostly the same assets as the core game.
In comparison to the core campaign storyline, which I felt was magnificent, Vessel of Hatred is an abruptly abridged product which offers no benefit toward replay value that you can't get from the core game.
Wait a few years and buy both games in a bundle, or grab the expansion for cheap. Diablo 4 will last as long as Diablo 3 (about 10 years) did so there's no hurry.