Turn the hero into a vampire in Ogre Battle?

You can't turn your actual Lord unit into a Vampire. However, any male unit that can become a Knight or Evil One unit is able to, with the use of an item called Blood Kiss. Naturally, if you want it early, you should do it with Knights, since Evil Ones are somewhat late-game classes. Knights require a at least 50 ALI on a male unit with decent physical stats, most of your Fighters in the early game will easily be able to class change into it within the first map or two. Evil Ones are low ALI units that promote from the Wild Man class, but it requires having fairly high stats so it's not going to be early.

Unfortunately, the only method to do it early is simply luck. The Blood Kiss, as a usable item, can only be found by random drops by enemies after defeating them in battle. You can't get it as buried treasure. So it's just a matter of being lucky enough to get one. Increase your chances by making sure you kill every single enemy on every map.

There is one guaranteed Blood Kiss, but it is extremely late game. It requires the character Galf to join your party (always one of the last characters), and you simply have to return to the Antanjyl map and visit the town Powant Noal with him in the unit.

Once you get one, just use it on a Knight or Evil One. For Knights, it's recommendable that you try to lower their ALI afterwards, since the Vampire only acts at night. There's no stat requirements or anything past actually being in the required class.