Welcome Kymco Owners!
Count me in - now there's three of us. I'm getting the feeling that there's something wrong with the Yager 200i! I've been through all the systems like you have. This Yager has only 5,000 miles and runs so rought. Unfortunately, for these machines, there's no diagnostic tools.
A stuck vent valve will not show on any diagnostic analyzer nor multimeter.
When my 2010 was misfiring at high rpm I replaced the coil and the counter (they both have to be replaced together) and has been running smoothly ever since, about five thousand miles. I'm up to 18K miles now and the scoot shows no sign of slowing down.The counter being the magneto that tells the coil when to fire.