I like the idea of trying to make them as different as possible combat-mechanic-wise from the undead versions. Just pondering what that looks like...
Well they both have the trait of becoming more vulnerable once they're "sated" on blood.
Let's see, what else is there…
The AD&D Vampiric Mist has a complicated blood feeding mechanism. At full strength it has 24 hit points (the maximum of its 3 Hit Dice), but it loses 1 hit points for every hour it spends without feeding due to "thirst," but it regains those lost hp at a 1:2 ratio when it drains blood.
However, this regaining of hp lost to its bloodthirst is NOT healing so a wounded Mist does not regain hit points by drinking blood, but recovers from injuries at the normal rate of healing.
Both the Mists are described as "enveloping" victims to drain their blood. The AD&D Vampiric Mist had a special mechanic for this:
Quoting
[Monstrous Manual (1993)…
A vampiric mist may attack one victim per round by reaching out with a gaseous tendril. Targets of a vampiric mist are treated as AC 10. Modify this number by the victim’s Dexterity, and by the magical protection worn ( + 1, + 2, etc.) but ignore magical shields, The touch of a vampiric mist drains 1d8 points of blood. A hit 4 above the needed to hit number means the mist has enveloped its victim. Enveloped victims are automatically hit each round until either the mist dies, finishes feeding, or retreats. Attacks by other characters against an enveloping vampiric mist divide their damage evenly—half against the mist, half against the victim. Only the enveloped victim may attack the mist without harm to himself. However, because of the disorienting effect of rapid blood loss, enveloped victims may not use any spells or magical devices that require concentration.
The Crimson Death is able to carry away enveloped victims as its feeding on them:
Quoting
[Monstrous Manual (1993)…
The crimson death is able to carry loads by levitating anything surrounded by its vaporous mass. Despite its vaporous nature, the crimson death possesses strength equal to a normal human. It is able to carry a passive weight equal to an adult human. If it attempts to pickup a struggling victim, it must make a successful attack roll for each round it carries the person. Failure means the person drops to the ground (suffering damage appropriate to the height of the fall).
The Vampiric Mist also sometimes divides into two half-health mists immediately after feeding to satiety. We could do something with that:
Quoting
[Monstrous Manual (1993)…
Vampiric mists reproduce via division. A mist is 10% likely to divide during its substantial stage immediately after feeding.The two mists created have 3 Hit Dice each, but only 4 hit points per die (thus they are born ravenously hungry).
