Under deceleration vacuum can go as high as 25 Hg which can pull oil past the piston rings and thru the intake valve guide. I don't know the condition of your engine but if it has a lot of miles the high vacuum could be coating the plugs with some oil which could cause a momentary missing condition. When running a vacuum advance shoot for 50-52 degrees total advance at cruising speeds. And then don't exceed 38 degrees mechanical advance. Remember, the vacuum advance doesn't work at all under full power and is there just for better fuel mileage.