you are probably best to use a phone card for both to be honest. money seems to go further that way. you can buy phone cards almost everywhere.
i am also on vodafone and it can be very expensive to make and receive calls in Turkey and the cost depends which package you are on. have a look at http://online.vodafone.co.uk/dispatch/Portal/appmanager/vodafone/wrp?_nfpb=true&_pageLabel=templateBlank&pageID=OS_0098 for more details and you will find out the exact cost.

if you wish to go down the Turkish SIM route, you have to visit a phone shop with your passport and get them to unblock your phone first - not too sure of the costs involved in this

A phone card from a phone box is the cheapest way to make phone calls. You could get a Turksih sim card but as Scots says you have to register the phone to be able to use it or your phone will get locked. It doens't cost anything to do but it will still be cheaper to use a phone card.