How come the elder vampires don't just explode each other's armies with the Second Secret of Dhar?
Who says they don't? It may be that skilled enough vampires can't bring armies against each other, but they disguise it as dueling for a point of honor because they collectively do not want the Second Secret to get out as a concept. That would also mean that when an elder vampire meets a younger one in battle even if the elder is caught by surprise and has no army with them they can still dismiss the army of their rival and then crush them with the advantages of age maintaining the hierarchy.