In the past, I've played a powerful PC and fought two vamp spawns at once, before. Alexandru Drachenfels and Vincent Stravokov, versus me playing Corvin Aegerfeld in the outskirts. Both MPCs used their mist form repeatedly to escape me. Each time they dropped into sight and I had the oppurtunity, I struck at them with various divine damage based spells, or went into melee and tore them apart. In between, they would come at me from behind, or bombard me with death and negative energy spells. Noone won (remember that I said that... noone "won.") the confrontation, and I'm pretty sure everybody on both sides walked away thinking they'd had a pretty epic fight, and were all the happier for it. I don't begrudge either of the MPCs their use of mist form, it made the encounter far more interesting, in fact.
Out of curiousity, I also checked the Vampire Spawn listing according the monstrous manual, to see if there was any real difference between it and a true vampire in terms of mist form. As a standard action, a vampire spawn can assume gaseous form at will, as the spell cast by a 6th level sorcerer, but it can remain gaseous infeinitely.
Its also not much of a difference from HiPSing, although it takes an action to use the widget, and we've had plenty of characters who used HiPS pretty often for PvP.
Far as I can tell, this is yet another of those ideal changes that is themed for winning. The MPC ability is lessened so that the encounter can be "won." It is not so much about balance, nor creative RP, simply winning a scenario. I'd suggest instead you focus on an IC means to deal with gaseous form... such as true seeing and a high spot / listen.