If you want to visit the Zambian side of the Victoria Falls, but you are staying in Zimbabwe, you'll need an entry visa for Zambia. Day visa are available and can be obtained at the border for $US20. To re-enter Zimbabwe you'll need a re-entry visa for Zimbabwe. Such re-entry visa, or multiple entry visa, cost $US45. If you only have a single-entry visa, you can also purchase another single entry visa on the spot when you return to Zimbabwe, for $US35. So-called "double entry visa" are also available, but they cannot, like all other multiple entry visas, be obtained at the port of entry into Zimbabwe.
TIPP: The Victoria Falls Bridge crosses the Zambezi River just below the Victoria Falls. As the Zambezi is the border between Zimbabwe and Zambia, the bridge links the two countries. If you only want to take a look at the Victoria Falls from the bridge, you can pass through the Zimbabwean border post at the Zimbabwean end of the bridge and walk ahead without any requirement for additional visas, though you will need your passport.