I've been studying a standard fuel injection system for a H booster and vapor carb application, and looking especially in all the spots on the intake manifold that accept gas vapors, for emission elimination, and am working those into my auto application. Some have major vacuum.

The air intake for any system is too open, and won't create any back pressure on the cell container, but if you apply it to a proper port, like, right after the line from your gas tank that collects evaporated fuel, then it has potential.
Old cars are different than new. If you run a new car and take the oil cap off the valve cover, it will not idle properly. It requires a vacuum even there.
I haven't researched this yet, but am curious to see what the affects are of reduced atmosphere has in an electrolyzer. Ambient air pressure is 14.7 psi. You drop a cell below that, could it weaken the surface tension of water?
I dunno, but its worth a look.
Some cars have smog pumps, and those might be useful in creating a vacuum, or just pumping hydroxy out of the cell.
Bzzz...