Did a bit of rewriting to tie these powers in together better. Let me know how this works now:
Blood Drain (Ex): A vampiric mist drains blood, dealing 1d3 points of Constitution damage on a successful tendril attack. The vampire mist craves blood, so it usually presses its attack until it can deal at least 8 points of Constitution damage.
On each successful attack, the vampiric mist gains 1d4 temporary hit points, which remain until lost from hunger or from taking damage. A vampire mist will reach satiation when it has its maximum of 9 hit points per hit die (27 hit points for a standard mist) and will disengage its attack. A vampiric mist that loses hit points from damage suffered must regain those hit points through healing and cannot regain them from draining blood.
Hunger (Ex): A vampiric mist begins to starve not long after feeding. For every 12 hours since it has last drained blood, a vampire mist will lose one hit point. When the vampire mist reaches its normal hit point total, it becomes ravenously hungry, and must search for a victim to drain. A vampire mist that reaches 0 hit points due to starvation dies immediately.
Satiation (Ex): When a vampire mist has drained enough blood to become sated, it becomes more substantial and its weight makes it slower. The vampire mist’s fly speed drops to 20 feet, its Dexterity score drops to 10, and it loses its damage reduction. These limitations persist for 1d6 minutes, at which point the creature starts to hunger again.