With Vodacom, just tell them you want the once off device payment deal. You then only pay the monthly package subscription and no handset installment on top of that per mont. The more expensive the phone, the move the once off fee will be.
Some SIM only contracts have higher voice/sms/data bundles than their "with device" counterparts. My wife is on a VC sim only and her bundles were doubled. I'm on MTN sim only and get similar bundles but a lot cheaper.
Also remember that on contract you only own the device at the end of the contract. At no time during that contract do you own the device, even though you pay a monthly fee.