The vent on each injector is there to normalize the pressure drop across the injector and to improve the atomization.  If the engine air inlet is very efficient the pressure in the manifold may be close or maybe even higher than the pressure around the injector, reversing the direction of air flow through the vents.  This is the case if the pressure recovery in the engine inlet is more than the pressure recovery of the cooling air.  Then fuel will either be poorly atomized or can actually be blown out the injector through the vents.  Some people have seen fuel stains around the injector, proof that this is happening.  The poor atomization can cause irregular fuel flow into the engine, creating a rough condition.  Some people have used the "turbo" shrouds around the injectors to source the bleed air to a higher pressure source, presumably the engine air inlet ahead of the filter.  This gives the best possible chance at sufficient atomization.  Others have run the vents to a "pitot tube" inlet ahead of the engine air cooling inlets.  Regardless, the idea is to get the pressure surrounding the injectors as high as possible.  Once I was thinking of ducting the vacuum pump outlet to the injectors - but didn't, as I wasn't sure about the relative air flow.  Within reason you can't get it too high.
Gary Casey