Yeah it will run like crap, and die when the vacuum line is disconnected. The best way to keep it running like this is to put your thumb over the end when not sucking up seafoam. Any truck will do that. The thing is with the 5.7 the vac hose only runs to 2 cylinders. It's better to suck the seafoam up through the pcv line. You treat all cylinders that way.
Edit:
Put mine in through the pcv hose