Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
The "vif" gene codes for "virion infectivity factor", a protein that increases the infectivity of the HIV particle.
The protein is found inside HIV-infected cells, and it works by interfering with one of the immune system's defences - a cellular protein called APOBEC3G. Basically what happens is that vif sticks to APOBEC3G and encourages the cell to degrade it, preventing it doing its job of sneaking into newly-formed virus particles and making them non-productive (see the APOBEC3G page for more information).
This has been verified in experiments. If you can create a HIV virus with the Vif protein missing (we would call this a "delta-Vif" strain of HIV), then it can still infect a cell - but the new virus particles produced from that cell contain APOBEC3G and therefore aren't very effective at infecting other cells.
Journal articles about Vif: