Vampire comes immediately after the fight with Doom's Wall. As Purim gets revived by her boyfriend Dyluck, Thanatos sends the Vampire to attack you. You should know that undead monsters do not like light. Well, you do not have light magic yet, so again, Popoi will have to stick to other means of magic. Vampire has a lot of HP, so you might be here a while. He also likes to put people to sleep, so beware. Again, Popoi's magic will do normal damage to the Vampire, and Purim should cast level 4 Cure Water if anyone's HP gets low.
You are nearing the final bosses of the game, so make sure that everything is at level 8. Lumina, Shade, and Dryad are your most important elements in the Mana Fortress, so they must be at level 8. Weapons need to be upgraded to level 8 also. Character levels should be no lower than 63.
It is Vampire again(as shown by the Japanese version where instead of having the name Buffy he is only known by the title Vampire Lord) - only he is gold. He is just as easy, though because you have light spells. Purim's the character of choice in this battle, so have her constantly cast level 8 Lucent Beams on the boss. Watch out for the boss's attacks and his spells. Popoi can help out Purim by casting any elemental magic, except Shade, preferably Dryad.