Always steam your tempeh before cooking for a less beany, more nutty flavor.
For a healthier version, use stevia instead of brown sugar and omit the oil.
In the US, regular brown sugar is not vegan. I suggest going with organic brown sugar, which is vegan.
If you're in a rush, you could skip the marinating and brush sauce all over tempeh when grilling. However, just take note that the tempeh will not absorb as much flavor as leaving it to marinate.
You might have noticed that I instructed to gently mix in the oil instead of rigorously whisking it with ketchup. Based from experience, I've learned that the tempeh tastes more flavorful if the fat is somewhat separated from the ketchup.
Tempeh is a soy bean cake with a mild nutty flavor. You could buy it at Asian grocery stores and health food stores such as Rainbow Grocery, Sprouts, Whole Foods, etc.