I'm not sure if the issue of class-level imbalance has been entirely addressed, but the two undead should be significantly behind in terms of 'class' power, which should open up weaknesses in the group. Nevertheless, the anti-undead suggestions all work if you want to make it plain to the PCs that their characters are becoming unbalanced, but I liked Stormborn's response: in-world, these undead beings should feel pressured, even if they are among the most powerful in the realm.
Other resistance groups should be 'reluctant' (some of them outwardly hostile) to horrors out of childhood nightmares taking a stance against the dark invading powers, even to the point of self-destruction. The other undead lords should be upset at the 'press' they're getting, and the dark forces should certainly make efforts to stop them.
I guess my point is that even minor tactical challenges can seem more daunting if the world feels less forgiving of mistakes. The vampire may be able to be healed nearly instantaneously, but has to return to his coffin, and the lich to his phylactery. Should either item become the focus of an enemy, or misunderstanding between allies, these are serious threats to their well-being. They're @15th-level, which makes them powerful, but it only takes a single paladin (even low-level) bent on retribution for crimes real or imagined to begin following their trail, sheltered by a world which won't side with monsters against justice, and the party would be put in a position of peril. Should he find the phylactery, or come upon them during the day, the party would be divided in how to respond, their own evil natures conflicting.