Oh my God, this isn't that easy to figure out. I'll just go with Micelus' suggestion, give the soldier to Reginald, no one else has any plan what to do with him other than spare him.
Your morality gets the better of you ( curse it! ) and you call Kevin.
"Kevin, wait, stop."
He turns back, relieved.
"Yes, boss?"
"Hrrmm...just leave him, I'll deal with him."
"Sure boss, whatever you say." he says, grateful that he no longer had to perform his "experiments" on the poor mercenary.
After Kevin leaves the general vicinity, you stare at the unconscious man, and think. How could he possibly be useful? You fed from him, gathered information, he's expendable. But maybe, just maybe, you do have some shred of decency, enough not to just kill and dispose of him.
But what will you do? After much pondering, you decide that sending this man to Reginald would be the best bet. Reginald wouldn't think much of it, but it doesn't matter.
Within the hour, the mercenary wakes up. He looks around, bewildered, until you appear.
"Where am I? Did you rob me, you piece of-"
"You passed out, I had no idea where you lived, so I brought you back to my castle."
He stops for a second, thinks, and seems to accept your explanation after noticing that he as all his stuff on him.
"Alright, I'd best be going now."
"Wait one second sir, would you like a more stable source of income?"
"What?"
"A better job?"
"Working for you?"
"No, working for a friend, well not friend, but acquaintance, of mine, Lord Reginald."
"Huh, I heard about him, some sort of glorified general."
"I'd recommend you not insult him, but if you do your job well, he may hire you, for a very long time, longer than your current job."
He thinks some more, seeing an opportunity.
"Alright, bring me to him."
Later
You bring the mercenary to Lord Jameson's castle in the north, and meet the "lord". You explain the situation.
"Why should I hire a mercenary? Mercenaries, I'll admit, can be good fighters, but they're not loyal. And my army won't be that much stronger with just a single new man."
"Yes, but there are other mercenaries, and if he gets into contact with them, he may tell them about what a good employer you are. For all we know, you might be getting a few dozen to a few hundred new soldiers within the month."
"Hmph...if you put it that way, fine. Not bad, vampire. Now get out."
You flit back to your castle, your cold heart just a bit warmer.
Sorry for the delay in updates, just made a very new paint adventure in Forum Games. Plot, plan, post. I'll reset the poll to something less relevant to the main story.