As CPWS Vodafone have all been migrated to Vodafone Direct, I would imagine that you would need to send the letter to Vodafone Direct. Unfortunately, I don't know the actual address... but it's best to check with them again because sometimes they give you specific cancellation references etc.
The way to minimize any loss of money from the 30 day notice period is basically to give your PAC number to your new provider approx. 10 days before the actual 30 days is up, leaving a few days spare in case something goes wrong (a port usually takes 7 days).
That way, you use as much of the line rental that you've already paid for. Of course, it's perfectly feasible that it actually saves you money to ditch the line rental paid in advance to take advantage of a better offer, so it just depends on what works better for you really.