No...no.. and no. If you wan't to kill something, grab a plane ya noob.
The 5 inchers on the CV are bad enough. I can't tell you how many times Iv'e been killed going 600 in a 262 and get hit by a 5 incher from a CV about 10K away. But we do need those to stop the Lancstucas from bombing the CV. We dont need 5 inchers on a field, terrible idea.
The auto-guns on the CVs are what are so terrible at killing you at unrealistic distances, behind hills, and what have you. I agree this needs to be fixed. The manned 5 inch guns rarely get you unless you "ask for it".
And it is important to stop "lancstukas" from bombing your CV, but unimportant to protect land-based fields?
It wouldn't effect vulching at all. Just like manned guns are destroyed, they would be destroyed if the attackers wanted to vulch. If you want to stop vulching you would be better off taking an osty or whirb.
Off-topic, but manned guns need to be stronger. This going out when you drop a sandwich on them is ridiculous. Either that, or we need a pair of VHs on airfields.
The flak guns would constitute a viable defense against high bombers, which fields, especially V-bases don't have. Don't speak to me of high-caps-most players simply aren't going to do it, buff hunting is less rewarding than hunting fighters currently since at the very least anyone with decent aim is going to take out your oil by being able to concentrate all the guns from 3 buffs on you. I can't imagine it being easy to hit any aerial targets EXCEPT buffs if the ranging were manual instead of proximity. Also, it would be good to have a manned gun that could be lowered and also be effective against tanks.
Imagine you were in your favorite fighter having a great sortie. Your in the furball having a blast with 8 kills. All of a sudden... BOOM! 75648975 shot you down.
I don't see how this would be good at all....
As I alluded to earlier, 9 out of 10 times this happens around a CV, it is an auto-gun that gets you.
Like I said before, If you want to kill something, grab a plane and learn to fight. There are always people in the TA willing to help ya out. 
Nothing you can grab will get up there and destroy a set buffs at 15K between the time they enter the sector and do their business, and anyone with decent aim will be more likely to destroy YOU. Nothing you can grab will be anything but giving a horde a free kill if the runway is covered and their are no guns up. There won't BE any guns up if bombers with a modicum of alt can destroy the VH, and anything else they want at will, which currently, they pretty much can.