ethereal entry?
The castle is parked over a huge swirling extra dimensional maelstrom that renders dimensional travel and conjuration magic impossible. Shifting ethereal would be suicide and/or one way trip to a random plane in the abyss. Messes with extradimensional/non-dimensional space as well.
witch's gate?
never heard of it. Hows it work? Note, they don't have a witch.
parachute on top and passwall?
Flying won't work because of the vampire's control of gravity for his castle. You'll fall, hit the zone between, bob up and down and slide off.
create water and bless under the castle so that it rains holy water up into the castle?
The vampire has absolute control over gravity. Coming from the bottom up leads to a similar problem as coming in from above and you are detected. Note, the detections are... impressive.
my personal favorite, use shrink item on a pond of holy water using the cloth option. have a really good tailor turn it into a really great cape or suit of clothes. offer it as a present, through intermediaries if need be. much later, cancel or dispel.
Actually, they were thinking of polymorph any object into gold coins, hope to be brought to his treasure hoard, but the wizard/cleric is finding the transmutation teleport a more viable option. Now hes trying to find a way to scry the guy so they don't have to search for him once they get up there.