Really Good Looking Tent, Dale!
I would not worry overly much about the vent. Unless you are placing it directly into the wind, it looks like it should work fine. The old Spectrum 1-2 had a similar scheme(except two outside vents supported by a small pole)and there were no problems with leakage. They were, however, way too small and that tent had some condensation issues. The Stephensons 2r places the upper vent on the front with an internal, somewhat adjustable, flap sewn in. As with most small tents in the 2-3 pound range, it too had some condensation issues.
The overall "tunnel" effect of the warm, moist air rising will mostly be lost, by the length of the skin over you. Soo, you really need some wind to drive ventilation to keep air moving. I was calculating an 8-10(~50-75sqin) in diameter upper vent to stay fairly condensation free...about the same or slightly smaller lower vents in ~1mph winds. This jives with what Mark F. was saying, too. The average hood opening is around 7-8" uncinched.
Good Luck with the tent!