Vampires are not immune to cheese. Just like anyone using HIPS, I'd expect if there is some kind of 'physical' interraction, much like someone attacking or touching another in invisibility for the illusion either spell or supernatural in nature to wear off... the mist form or the PC attempting to do so would show themselves, at least temporarily so that players would know completely what the situation was, as well as PCs who are well learned in such lore could possibly ATTEMPT TO ROLEPLAY BACK, and identify what was going on with proper checks or professional expertise.
I still stand by my statement that this was UNINTENTIONAL grieving...which really there is nothing wrong with, considering we have yes all made mistakes in the past, and what we do is learn from them.
Cheers to whoever this thread is about, learn from this, and game on.
Reeeeeeelaaaaax....