I have seen folks use coconut oil as a substitute for grass fed butter (or organic butter, or Irish butter or many other variations I have heard). Almost all of the substitutes I have heard are fat sources designed to get fat based calories into the coffee. The vanilla bean powder you have heard about adds almost no calories, but will certainly add flavor. It accomplishes a very different goal than what the Bulletproof folks are espousing.
Keep in mind there are any number of things you can add to coffee to make the caffeine more palatable. There is an entire industry surrounding this. What you really need to ask is what you are trying to accomplish by adding something and find the best ingredient for your needs. Low or no calories flavoring agents maybe suited if you are just not into the taste of coffee. If you are looking for a fat calorie source, experimenting with different oils until you find one that fits your needs and has a flavor you can deal with may be the way to go.
Personally I look for more protein and not as much fat in my diet. I like the taste of coffee. A plain cup of cone coffee and a serving of organic Greek yogurt accomplishes what I am trying to do just fine and doesn't require breaking out or cleaning a blender.