This will be an immensely divisive issue among the clans presently allied with him! The Galatath and Laethan would have a historic bone to pick with the Aellyn, the Galatath in particular, who suffered fierce territory wars with them and were literally magic-cursed to be homeless wanderers for two generations. The Laethan simply hate the magic-users on principle. The Orminairi are primarily pacifistic, but would note that the vampire situation with the Laethan may have been devised by the Aellyn (who knew of Legolas' quest) to try and wedge them away from joining the cause of unification.

Legolas' advisor from the Aellyn who was cast out to remain would advise as an aside that war against them would be costly in lives, but that not listening to the native cries for war would no doubt cause trouble: better to make a very small formal contingent of those who want to fight and are of age and give them one's blessing. It's a gift of men and banners, but won't carry the future of the newfound kingdom on its shoulders.

Additionally, some of the Laethan would rally additional support from other tribes to ask for a formal blessing on routing any last vampires from the Greenwood and villages within it.