The Zimbabwe/Zambia KAZA UNIVISA is available at Victoria Falls, from either country, for US$50 per person. The UNIVISA is primarily aimed at tourists who will cross between these countries, either from airport to hotels or for activities across the border, be it once or multiple times - it is meant to simplify the process and avoid multiple visa purchases. You can cross between these two countries as many times as you like within a period of 30 days. It does not include Botswana.
You need to ask for it at immigration - otherwise they may simply give you a normal single entry permit. It is best to have US$50 available in cash, clean current notes, although credit card payments are sometimes possible.
Yellow Fever Vaccination Certificate is not required to enter Zimbabwe unless you have traveled to a high-risk region in the last 6 months or are coming from a country listed as a transmission risk.
www.zimimmigration.gov.zw