"You need to give 30 days notice"
is there some 'technical' reason for that? or is it just a way for them to grab more money?
I spoke to VF today and asked about this again.
You are billed for the month ahead so each month you are billed on 1st day. If your transition to other ISP takes place on the 2nd day of that month, then unfortunately you will be billed by both ISP for the whole month.
That's why it pays to arrange the move towards the later part of the month so as to avoid this. I'm moving to Snap around 20 July. Think 10 days should be enough for the transition to complete.