Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
vpx is found in HIV-2 (and SIV), but not in HIV-1. It is closely related to vpr (if we compare their genetic sequences), which indicates that its existence might have come about as a duplication of the vpr gene.
Its role in the life of HIV is not entirely clear! It certainly seems to be "dispensable", since types of HIV-2 without a functioning vpx gene still seem to be able to replicate and to infect cells.... However, it seems that vpx does have some effect of making viral reproduction more efficient, especially in non-dividing cells such as macrophages. The molecular mechanisms behind this are not yet fully understood.
More detailed information can be found in these journal articles: