The phone is brought in without the company's knowledge and not officially tested.
The phone does the basic stuff we expect - voice calls, video calls, SMS and browsing.
Someone inserts a SIM card in it and it does funny things that owner is not aware: it connects to a Chinese server every hour to check for weather updates.
Or it has a bug in the firmware that makes it connect every ten minutes to a Taiwanese e-mail server even if no e-mail account is defined.
How do you know the phone is not doing this?
And how is this possibly a Vodafone problem? The phone asks for access to data, Vodafone provides it and charges for the service.
Certainly is wrong for a Vodafone store person to say "It happens like this, Vodafone charges differently for imported handsets" because this is not the case.
The fact is we don't know what the phone is doing.
If people buy an imported handset this is the risk you are taking.
So yes, the store is responsible for warranty and to make sure the phone is fit for the purposes. This is not Vodafone's problem.