i-phones and Vodaphone are very popular, so I'm sure that there'll be someone in your home and work areas that will have an i-phone, will be with Vodaphone and quite likely someone with both.
Have you compared experiences with them eg. does their phone indicate reduced signal in the area(s)?
I've had a similar experience with O2.
My phone (HTC HD2) has had numerous things done to it (they won't replace it) but it's not the best (I maintain that the antenna is naffed). As soon as I could I changed to SIM-only, waiting for 4G and a move to a more customer-oriented company.
Anyway, the first thing to do is to (try to) prove that you have some fault, hence try to get good circumstancial evidence that Vodaphone's lack of adequate provision of total service (ie. phone and coverage) is specific to you, which would prevent them quoting the 'coverage cannot be guaranteed' clause. (This is because if the general coverage is OK then clearly the phone must be at fault in some way, and they are contractually obligated to provide one that works properly).
Hopefully you'll find someone with an i-phone, in one of the areas and they'll have no problem at all, thereby pointing the finger at the phone.
Can I ask one question - are you left-handed?
Or another - do you hold the phone in your left hand?