After I pull the meat out of the sous vide and then use a blow torch to sear it, should I use a fat on it or not? Will that get a better crust on it?
I have not noticed any real difference from using a fat, so I skip it. What does make a difference is thorough drying the meat.
I haven't ever had great results from a torch though. I prefer a screaming hot pan or grill, although both are way more effort.
At least one commercial chain (Panera Bread) uses fat when searing meat that is prepared sous vide.
Here is a video example. After cooking sous vide, they dry the meat and use a very hot cast iron pan with generous amounts of butter.
I haven't noticed any real difference with or without oil or fat to be honest. When searing in a pan, the oil would simply aid heat transfer from the pan to the meat by eliminating insulting air pockets.
However, I have experimented a little with brushing a weak glucose syrup onto steak before torching, and believe (without scientific measurements) that this results in a nicer crust with a lower propensity to blacken due to the flame. This is based on something I read in McGee's On Food and Cooking, which implied that the browning Maillard reaction would be accelerated by the addition of glucose. Apparently glucose is more reactive than sucrose found in normal sugar.
I've also discovered that drying the surface of meats with a paper towel just before blow-torching results in nicer and more rapid browning. I'd speculate this allows the surface temperature to rise more quickly as there is less liquid to boil off first.