Can't resist peeking behind an open door!
As to the priority tasks, coding and map design aren't really my sort of thing. The historical research, AI behaviour and mission types are, so I'd prefer to focus on those.
Determining the general nature and feel of the campaign seems the number one priority to me (also see my post in storyline) since that will be our take-off point and determine the direction of a lot of the other work to be done. Think about type of force (regular army, marines, airmobiles, etc), period (early or late vietnam war or something in between), location (where in Vietnam), etc. Post your ideas and views in the storyline thread and we will see what evolves!
Remco