Heat can be just as big of a problem as honey. Keep the vac out of the sun and ventilate them (an provide water) as soon as possible. That is why I made the slide off top on mine. On hot days, I pour water into the frames before I start.
If they overheat and start dying, you will end up with a wet mass of dead bees. Trust me, I learned the hard way. My original bee vac had plexiglass windows and it didn't take long in the sun for it to turn into an oven :-\