Download GPU Tweak II and use the option to update the vBIOS. If none is found, it's because your GPU is always running the latest vBIOS.
Where is this option to update the vBIOS in Tweak II? I've never seen it. Unless you mean when you click the cog icon [left of Minimise/Maximise/Exit] there is the Live Update option to choose how to handle updates, I can not find a specific option for vBIOS update. But an educated guess tells me that this is the option you are referring to, and that if there was ever a vBIOS update from ASUS, Live Update controls the file download according to the settings you choose in the Live Update options.