Is this vent system ok?
I have never installed one yet, and don't plan on it, ESPECIALLY if the situation is as simple as you're describing it.
I am a plumbing apprentice in Oregon, and while the code does not specifically allow them, it also does not specifically prohibit them.
What gets me the most is that our code requires vents to be designed to allow free flow of air throughout the system at all times, but some inspectors still allow these things to be installed. By my interpretation, that section of the code is saying that these devices don't quite make par, but overall, it's up to the inspector.
Also, keep in mind that he may or may not be comfortable installing the flashing post-shingle, so he may request that you get a roofer to come install it.