I think it adds a greater depth to Vader's character when he is portrayed as constantly pondering the Emperor's demise. I find it hard to believe that he would simply put his plans on "hold," especially when Galen clearly could have helped him destroy the Emperor, as he nearly did. Vader blew it, I think. He had the one person who was both powerful and evil enough, and he let him slip away.
"Everything I tell you is a lie. Every question I ask you is a trick. You will find no truth in me."
―Vergere to Jacen Solo