I have had similar issue with VP12 before.
You could try opening a new empty vegas project, then diabling view->project media window, and restart the problematic project now without the 'project media' window. So disabled.
Vegas remembers the last setting so disabeling the window is only possible via a project that 'does' open, in this case an empty one.
So when you open the problematic project after it Vegas remembers the previous setting.
Sometimes with many media present the 'project media' window causes problems. Also the window itself loads very slow when you enable it.