The RVP of hydrocarbons and oxygenates should actually blend very linearly, with the exception of the lighter alcohols, methanol and ethanol. Methanol is not expected to be used as an oxygenate due to its high RVP, and there are accepted rules of thumb for approximating the RVP boost from ethanol.
Regarding distillation, EPA's complex model uses the percent evaporated at a given temperature as opposed to the temperature at which a given percentage of fuel evaporates. This was done in part to avoid this problem, since percent evaporated, like RVP, blends very linearly.(7/1/94)
This question and answer was posted at Consolidated List of Reformulated Gasoline and Anti-Dumping Questions and Answers: July 1, 1994 through November 10, 1997 (PDF)(333 pp, 18.17 MB, EPA420-R-03-009, July 2003, About PDF)