To enter the gates to the hall you'll need to visit at night. Enter through the gates and a small scene will take place. Head north up the long set of stairs to the entrance of Vinquette Hall and enter the building. Inside you'll meet with the Ebony Butler and Isabella who'll offer you to stay the night. After a long conversation you'll arrive in your sleeping quarters. Open the chest here for an Angel Grail then exit the room. Go right and enter the room to talk to Isabella. She'll have a nice little chit-chat with you and tells you of a water spirit trapped in Marsh Cave. Head back to your room and the boy will go to sleep. The heroine will wake up in the night and will be free to venture on her own.
Leave the room and the heroine will witness the Ivory Butler talking to himself. Save at the Mana Goddess then go down the stairs and enter the room to the lower-right.
Vinquette Hall (Dungeon)
This is a good time to level up your magic as most mobs here are only weak against magic damage and it's your most safest way to get through them. Have your staff equipped so you can kill them without having to be infront of them. Head to the far lower-right corner and open the chest to obtain a Light Coin, then head up though the door just ahead of you. Go around and down the stairs, then head down and enter the first door here. Recover your HP and MP and save at the silver Mana Goddess and open the chest here for a Magic Walnut. Go back out of this room, head left and enter the next door. Destroy the Metaball in this room with the bow to reveal the fire spirit Salamander. Salamander magic is only effective vs Batmos, Zonbines and Skull Beasts, so use it when you can to level it up (remember to equip the staff first.)
Return to the very first room then head up, far left, and enter the door here. Kill the mob in this room by running out of its range and using Salamander magic. Exit the room through the door to the north, then head up, left, and enter the 2nd door. Go around and down the stairs, left, and enter the first door. A small sequence will take place with the Ivory Butler who then turns into a werewolf and attacks you. This boss is not weak to anything so you're best off conserving mana for heals and attacking him with your bow. Keep at max distance and just spam shots at him. If he gets close then run away and heal up every time he uses his special attacks. Since you're only doing 1 damage per hit he will take some time to down. Once he is dead go through the door to the upper left, go round the stairs and south, down, far left, and enter the 2nd door. Head up north through the door to a golden Mana Goddess. Save here as the next room leads to the next boss, Count Lee. If you want to gain a few levels before facing him you can repeatedly go back and forth between rooms defeating mobs and recover HP and MP until you reach a level you feel comfy with.
Once Count Lee is defeated you'll be faced with yet another scenario where the hero and heroine finally get outside again. Head south twice and purchase the Cactus Essence from Niccolo if you haven't already, save here, and go south once more to reach the Scaly Lair.