If it is like the one on my Ryobi trimmer it sucks fuel from the tank, through the carb. If yours does work that way the fuel filter (on mine...in the tank) might be stopped up, trash in carb, or a fuel line crimped causing the problem, as you suspected from vacuum. You could disconnect one line (the one from the carb) to relieve it. If you remove the line you might shoot some aerosol carb cleaner into the carb through the tube. This might dislodge or clean the carb a little, but I would pump some fuel through it pretty quick. It might damage the diaphram or o rings if left full strength for very long.