I have a 07 ranger, 6 cyl. The error code says I have a leak in the vapor system.

what size leak does the code say? the canister vent solenoid should be open to atmosphere all the time until it decides it wants to pull a vacuum on the evap. system then the pcm should close the vent solenoid and open the vapor management valve which allows engine vacuum to be applied to the evap. system it will set a leak code when it cannot achieve the amount of vacuum it want's.when you performed the smoke test you should of had smoke coming from the hose that the vent valve connects to since it is normally open unless you pinched the hose or manually closed it with a scan tool. if you didnt see smoke coming from any lines then i would suspect the vent solenoid

Thank you very much for this answer but I don't know enough to know where these components are located. Is there some where I can get pictures and/or diagramphs of the system. The smoke did come out of a canister behind the gas tank on and off but how do we know if the system is working?

No sorry i dont know a good place to find any diagrams on it. Its hard to diagnose when you dont have a scan tool so you can actuate the solenoids and test the system. Its not gonna hurt anything if you drive it like that. You could take it to a shop and pay the diag. fee and replace the part yourself. But i really dont think vent solenoids are much money and there easy to replace and thats most likely what it is. I wish i could help ya but there is no good way to test it without the right stuff

Computer said it has a GROSS leak. We saw the smoke start and stop comming out of the back canister vent, so I assume a selenoid is working. But is it working at the right time. The reason I questioned if I have a problem is the red gas cap indicator on the dashboard is on. Replaced the cap no effect. Cleared the error codes and it came back. Also I'm only getting 14 MPG and the truck has 35k on it.

gas cap

Since you replaced the gas cap its doubtful its the gas cap rick. You have no way of telling if the vent is closing and that could cause a gross leak its a big opening and could turn that light on. Replace the solenoid and go from there

The canister vent valve (solenoid) is a common problem on Fords. I would be willing to bet that it is where your problems lies.