I'm somewhat confused regarding the contractor's statement that your ventilation was "adequate." From what your describing it would not be adequate. Obviously the sheathing is blocking any air movement to any degree. Further the perforated vinyl siding pieces are not sufficient in themselves. I recommend you contact another contractor or handyman type person to install proper louvered vents in your gables. These vents are readily available in several sizes and shapes and will be easy to install through the sheathing that is already in place. The perforated vinyl could even be left in place and just cut through as needed and that will leave a nice finish look from the outside.
Dennis (cut2it)
some additional information.
I have an "L" shaped home. On all sides of the home I have continuous perforated soffit. On 1 of the gable ends, I have a standard slotted gable vent. On the short side of the "L" I have ridge vent. On the long portion of the "L" I have approx. 7 turtle shell type vents. It's the 2 gable ends at the end of the long portion of the "L" that I'm inquiring about.
Thank you very much Chris for this additional information. I withdraw my original answer. The contractor was correct. Your system does have adequate ventilation. The sheathing behind the perforated vinyl was merely to give is something solid to go against. Since no additional vents would have been needed. Unless you have some other problem like condensation build up the system you have in place is just fine.
C&onstructi&on C&onsultant
Over 40 yrs. as General Contractor for new const. & remodel - retired with time for you.