Can a plumbing vent start in the horizontal where it connects to drain line?
The only thing I see in the code is “A dry vent connecting to a horizontal drain shall connect above the centerline of the horizontal drain pipe.”, so as long the wye is angled up (which it would be to get the slope you described), I think you would be good. 

I am not a plumber, so I advise waiting on some more experienced advise.