Vampire Charmed Mechanics

So I had a recent session where I had a disagreement with the DM on how an enemy Vampire's ability worked. Essentially I started combat with low initiative, and before my turn comes I fail a wisdom save and am charmed by the enemy Vampire. The DM said when I'm charmed by it, I must take actions most favorable to his orders, and the vampire pointed to a specific square and told me to hold that ally down. He essentially wanted me to be in flanking position while trying to grapple my ally. 

So when my turn comes, I fully intend on complying, but I also ask if I can do something to the enemy. I was playing as an Illrigger lvl 8, and I asked if I could place one of my seals on the vampire as a bonus action. Essentially placing a seal considers him 'interdicted' which has interaction with other abilities, but also when he takes damage I can burn the seal(s) attached to him and do some damage. I said because it's an ability that doesn't do damage unless I choose to burn them, it shouldn't violate the charmed condition. The DM said no, reasoning that the only reason to place it in the first place is to do damage. This isn't technically true, but even if it was it's still an ability that doesn't immediately or inevitably do damage. I didn't argue further though. 

When I went to do the grapple check, my ally made his saving throw so it was unsuccessful. I then told my DM I was going to use some remaining movement to step away, but the DM objected. He said I was ordered to stand there and hold him down. I said I attempted and failed, and there was no reason I couldn't move back since I can move back and try again next turn. The DM said it doesn't make sense that I would try, fail, and move away. That's not 'most favorable' in his book. 

Then during the Vampires turn, the Vampire does multiple attacks on my ally with advantage. I ask why he has advantage, he says because he's flanking with me (we were playing with flanking). I told him this shouldn't give him advantage because I'm not his ally, I'm simply charmed by him. He responded "I'm ruling that he does because you're trying to grapple him." I don't agree that this automatically makes me his ally for purposes of flanking, so I responded "I just want to make sure that this charmed condition doesn't evolve into dominate person." He responded saying that it's fine because he's only making me grapple my ally, not attack them. 

There were other things I wanted to try later, like the vampire summoned allies, so on my next turn I was gonna attack an ally then try to grapple my ally since I have extra attack. I never got the chance, however, because I was able to make a save by my next turn. 

So I was wondering if I'm right about this. Did my DM incidentally buff the charmed condition? Or am I mistaken? 

I'd say you are both mistaken.  The Interdiction would be similar to handing someone a live bomb with a remote trigger.  No, it doesn't hurt them immediately but the intent is there.

As far as being counted as an ally, that depends on whether the vampire can use a social interaction (which they have advantage on) to convince you that it's the right thing to do.  Deception or Intimidation perhaps.  

Charmed [Condition]

While you have the Charmed condition, you experience the following effects.

Can’t Harm the Charmer. You can’t attack the charmer or target the charmer with damaging abilities or magical effects.

Social Advantage. The charmer has Advantage on any ability check to interact with you socially.

Charmed is not dominated. A charmed character would not attack their allies just because someone told them to. You just wouldn’t attack the vampire. You may even help your friends, like cast bless on some allies or something, but you wouldn’t attack them. You could freely attack the vampire’s allies. The big point of the vampire charm is setting you up for a bite you wouldn’t resist. It does not make you change sides in the battle. 
Now if the vampire had used dominate person instead of charm person, then it could order you to attack an ally. But off-the-shelf vampires don’t have dominate, so that seems unlikely. 

This assumes they are using the standard monster manual vamp, and not a 3rd party or homebrew version.

I’d agree with your DM about the interdict. I realize technically you aren’t harming it, but that really seems like splitting hairs.

Quote from ArmorStorm >>

I'd say you are both mistaken.  The Interdiction would be similar to handing someone a live bomb with a remote trigger.  No, it doesn't hurt them immediately but the intent is there.

As far as being counted as an ally, that depends on whether the vampire can use a social interaction (which they have advantage on) to convince you that it's the right thing to do.  Deception or Intimidation perhaps.  

 

Charmed [Condition]

While you have the Charmed condition, you experience the following effects.

Can’t Harm the Charmer. You can’t attack the charmer or target the charmer with damaging abilities or magical effects.

Social Advantage. The charmer has Advantage on any ability check to interact with you socially.

Note that depending on iteration, Vampires have a lot of additional riders on their Charm power. To wit:

2014

Charm. The vampire targets one humanoid it can see within 30 feet of it. If the target can see the vampire, the target must succeed on a DC 17 Wisdom saving throw against this magic or be charmed by the vampire. The charmed target regards the vampire as a trusted friend to be heeded and protected. Although the target isn’t under the vampire’s control, it takes the vampire’s requests or actions in the most favorable way it can, and it is a willing target for the vampire’s bite attack.

Each time the vampire or the vampire’s companions do anything harmful to the target, it can repeat the saving throw, ending the effect on itself on a success. Otherwise, the effect lasts 24 hours or until the vampire is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect.

 

2024

Charm (Recharge 5–6). The vampire casts Charm Person, requiring no spell components and using Charisma as the spellcasting ability (spell save DC 17), and the duration is 24 hours. The Charmed target is a willing recipient of the vampire’s Bite, the damage of which doesn’t end the spell. When the spell ends, the target is unaware it was Charmed by the vampire.

While you're not really required to comply with instructions for 2024, it does say you're a Friendly for the Vampire and I'd say blocking the use of combat/debuff powers is within the RAI scope of the Charmed condition. And the 2014 is pretty close to a full Dominate effect.