But I'm doing a second playthrough with the objective of making every item I can no matter how hard it can be. And for that I need a Blood Jewel. I've attacked the vampire from every direction with multiple setups and nothing. I believe I've never broke a single Vampire core.
I really want to break one just to see where the bloody thing is.
IIrc (though it has been a while since I played) the core is below the head, covered by the wings. I suggest to careflully take out the wings first. Then attack the head preferably with sweep dive) till the vampire is close to death. Then restore AP to 100, if possible also activate Might Reinforce. Give the vampire a deadly hit from behind with sweep dive, and then attack him more and restore AP with some Light Warriors. Even better would be if you can manage to lure the vampire onto that little platform within the water on that battlefield. IIrc the core had relatively high HP, so you might want to have Heroism, a slayer skill and Fists of Iron active to deal enough damage to break it. The party I personally suggest would be Zunde, Mithra, Celes and Fraudir. Mithra with Lightning Bolt, Zunde with Sweep Dive and Dreaded Radius and the 2 others with ground combo attacks such as Resonating Pain and Mist Phantom. The LWs and Mithra should have Spirit Control if possible.