Alright, well my guy just got killed off in the area right after the Royal Chapel. I don't know how the Vampire Killer class is supposed to deal with the Panther Skeletons - they seem to have some serious problems in the mid-game, despite being really easy to start out.
Here are the problems I see:
1. Weak weapons. Vampire Killer is really good at the start, but the lack of improvements later on means it will be surpassed by the other class' weapons in time. The Thorn Whip and the Lightning Whip help, but they're not enough. Maybe this gets resolved later on, but I've never been there.
2. Crappy armor. You're stuck with the vampire killer armor for the long haul, and it's pretty bad. The class's high stats and the whip mean that it doesn't hold you back much at the start, but once those things' usefulness starts to fade away, it definitely shows. Again, maybe you can upgrade the armor later in the game, but even if you can, you have to go an awfully long way with extremely basic armor, and no shield ever.
Of course, giving Vampire Killers better starting armor would just make the first few areas too easy.
3. The lack of a ranged attack and a heal. Mystic weapons give you long ranged attacks, and the holy water mystic weapon lets you heal yourself, but you really have to choose between one or the other, and once your whip and armor troubles start showing up, you're going to need both. Maybe giving Vampire Killer's more special abilities earlier in the game, so they can access the Soul Ice ability sooner would help resolve this.
One thing I thought I'd bring up on the other end of the equation is an item that makes some parts of the game too easy - the orb of recall. I think it would be much less of a 'get out of jail free' card if there was a delay of a few turns in between using it and showing up safely in town.