the vacuum is exerting a lot more force than the air escaping from the balloon. if the vacuum was placed in line with the wheels, instead of perpendicular to it, it would pull the car across the board. it might even suck the whole straw into the vacuum hose - which would then make it obvious that the vacuum is exerting a force on the car, right? well that force doesn't disappear when you turn the hose sideways -- it's just pointed in a different direction.
what's happening in the video is that the force of the vacuum cleaner is overcoming the force exerted by the air in the balloon.
"vacuum" in space is not the same as what happens at the end of a vacuum cleaner hose - it's emptiness, an absence of air. at the end of a vacuum cleaner, there's not an absence of air -- their's air moving with a lot of velocity into the vacuum hose. so this guy is not testing whether a balloon can exert a force without air to "push against" -- there's plenty of air still behind the balloon. it's just that that air is moving quickly into the vacuum hose instead of just sitting there. if you really want to test the theory the poster of the video is pushing, you need to do this in a vacuum chamber - a place where there's no air. you want the absence of other forces -- but in this video, we've added another force, one that's bigger than the force from the balloon.
do you know how airplane wings work? air moves over the top of the wing faster than the bottom, so there's lower pressure on top, and the air underneath effectively pushes the wing upward (because the air above isn't pushing as hard downwards)? the same thing is happening at the back of the car in the video. there's very fast moving air behind the car, faster than the air moving out of the balloon, and the air in front of the car isn't moving at all. so there's a low pressure pocket behind the car - like what you have above an airplane wing - and that's pulling the car backwards and sideways, with more force than the balloon.
