Does vanilla powder really prevent melted chocolate from seizing?
I just reviewed the Nielsen Massey website and under their FAQ's they suggest that vanilla powder be used for "liquid sensitive products".  The powdered nature of the vanilla would allow you to add it to melted chocolate without causing the melted chocolate to seize up.  While vanilla has a unique and characteristic flavor of its own, it also helps to heighten and intensify other flavors, chocolate being one of them.

When chocolate seizes the emulsion of cocoa butter and cocoa powder has been interrupted  by the introduction of a small amount of moisture. You either need to keep all liquids out of melted chocolate or add in a significant amount.

So...it only prevents it from seizing if you were going to be adding vanilla in extract form but no other liquids.  It would not be something you add as a preventative measure.

Prevent the seizing through proper procedures in the first place:


Do not allow water in the water bath to touch bottom of the bowl that you're melting chocolate in.
The steam produced by the water is what's doing the melting so as long as wafts of steam are rising from the surface it's hot enough.  The water doesn't even need to be simmering - cocoa butter starts to melt around 83-85 degrees F.
Make sure all utensils are dry so water droplets aren't introduced
Make sure the bowl you're using is large enough for the edges to flare out beyond the edge of the pot.  If the edges just meet the edge of the pot then steam will condense on the edge and get into the chocolate causing it to seize.