Small vacuum leak and evap system

I have a 2002 mustang GT 5 spd with a vortech v2sq and an anderson powerpipe. Here is my problem: My vacuum at idle is on the low side 16-18 mmhg. I tool the car to a shop and they tried to find the leak, the only thing they found is when they disconnect the vacuum hose that goes to the evap solenoid (in the front passenger side fender) the vacuum go to 21mmhg (a "more" normal value)! But they didnt find any vacuum leak in the evap system (smoke test)! They told me that it was normal for the evap system to drop the vacuum but every mustang I saw with a motor like mine (no big cam, no ported head) and a blower had 20-22mmhg.

Now the questions:

Is it normal that the evap system drop the vacuum?
What can cause this leak?
Can this be the source of my P0172 and P0175 cel? (they keep coming every 2 days)?
Could this be the cause of my screwed STFT? (the car keep trying to lean both banks because they are running too rich...this happens only at idle and low throtles)

Thank you,

The shop did a smoke test on the evap and found no leak!

I don't know if this can help you, but at one point the shop inject smoke in the evap test port "at least that's what the guys said he was doing" and they plugged a snap-on tool in the obd2 port to activate and disactivate the evap system... at some point smoke was coming out of where you put fuel (I say where you put fuel because because it wasn't from the filler cap (since it was on))!

Than you for your help!