I'm not too familiar with the Vision system, but I've made several million MAP sensors (only a few came back.  Honest...).  THere is almost no way the sensor itself could fail that way, so I would almost guarantee there is blockage in the line or port.  They probably use a small MEMS sensor more likely than not the same one you can buy from the Newark catalog and it has a very small pressure port.  The might have positioned it close to the outside of the box so that its port protrudes or it is inside connected with a short hose that could be kinked.  If the sensor port is exposed you can look into it, but shouldn't stick a wire in it as the sense element might be the first thing the wire hits.  Very fragile.