I just tried testing US-Vichy naval interaction.
What I found was a bug. Neutral Vichy forces were allowed to intercept moving US naval forces, and US forces could intercept Vichy forces, but neither the US nor Vichy were allowed to initiate combat in the choose combat portion of the sequence of play. Furthermore, far worse, if Vichy successfully intercepted, and the US chose to fight through, or vice versa, a MadExcept was generated. If you told the game to continue, the game got stuck in the Naval Intercept Digression: Naval Combat Sequence, with no way to proceed.
Thus, at the moment, the US and a Vichy that is neutral with respect to the US can not fight in MWiF. I have only tested what happens if there are only US and Vichy forces in a sea area, with no units from other countries.
I suppose I should write up a bug report about this, since a MadExcept was generated, but I really don't feel like doing so at the moment.
Someday I will test what happens if Vichy forces are in the same sea box with other Axis forces that the US is at war with, and see what happens then.
I thought I knew how to play this game....