I have Sony Vegas 12 and installed version Vegas Pro Production Assistant 2.0 build 9 (2.0.9) on my windows 7 professional pc. Production assistant does not open from withing vegas pro 12, but will open from start menu.
I removed the software and reinstalled with not change.
Make sure that the Production Assistant windows isn't just a thin window edge next to the Trimmer. Vegas Pro 12.0 has a bug that causes this to happen with all command extensions: