Which skills are best for a hero?

Well, please tell me, if it's not too much trouble.
Does the hero's morale affect the probability of joining?
Approximately how much stronger should the Necromancer's army be in order to recruit a group of Greater Vampires?

The chance of joining does not increase with the hero's morale. The higher the morale, the stronger the creatures are, as calculated by the computer, and the higher the probability that they will flee. However, the disposition of the neutrals is what is determined at the beginning of the mission, and this parameter has the greatest influence on the chance of joining. And the disposition, in turn, depends on the Diplomacy skill and the Crown of Leadership. Of course, the factors you mentioned above also have a direct impact on the chance of joining.
To have a sufficient chance of joining, the army must exceed the neutral force to the point where the ratio of forces exceeds the combat threshold. Or at least the ratio of forces exceeds the joining threshold. To avoid getting bogged down in formulas, I'll say that I recruit neutrals when the danger level is very low.
Furthermore, regarding the skill itself. Taking Leadership early or mid-game involves a great risk. It's a different story in long games. And it's not necessary to develop Leadership with the main hero when you can develop a secondary hero through mentoring.