Probably not your problem, but I thought it worth mentioning for the smaller B&S engines. I've had problems with the governing due to an air vane getting too tight or stuck once the spindle is tightened down on to the coil. The only solution I've found is to file or sand some of the plastic off the bottom of the spindle until it swings free again. When stuck the engine runs just as you say, it just goes flat out and won't idle down. Another quick fix is to loosen the bolt on the coil a bit that holds the spindle, but then you could risk the coil coming loose.
I find a lot of the newer B&S are pains to get to idle e.g. Sprint series. As their name suggests, they do just seem to want to keep sprinting ahead rather than tune back to an idle. Some people have had good results by bending the arm that the spring attaches to or manually stretching the spring. Perhaps a slightly longer spring might do the job in the case of the much larger Vanguard engine? Just a thought.
