A Haynes manual has a diagram of the vacuam lines as they distribute through the vehicle. If you take the color code information given by the pro, you can start by plugging testing them with your pump to find which leg the leak is in, trace down from there.
If it's the locking circuit, which it probably will be, you can start at the passenger door and determine if it is source or destination at that point. If it's supply go to the drivers door, if it's destination go from there to the rear passenger door and on to the fuel tank door.
By splitting it in half and seeing which way to go, you will statistically decrease the number of door panels to take apart.
Of course, if your luck is like mine, it doesn't matter where you start, because it will be the last possibility that will contain the problem <g>.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in