I want to make compound butter using my powerful blender. I am having trouble doing this with solid or soft butter because the blender cannot create a vortex and mix as well compared to blending with liquids. I was thinking of melting the butter and adding ingredients (e.g., shallots) and blending it into a beautiful liquid, and then refrigerating it. What would this do? 
I think its possible, but because you can't fit a lid on top of whatever pot is holding the water (well, not easily; you can do some cutting to make a hole for the sous vide device in a plastic lid, but then that lid/pot become dedicated to the technique), and that lets the heat escape probably more than what would be desirable for what you're trying to do.  What you really need is something more like this.  It controls the water temp to perform the function of an immersion circulator for sous vide cooking, but it can also be used for slow cooking without the water.  I searched Amazon for sous vide slow cooker and got lots of results. 