My knowledge too is limited, but I`m sure I can learn fast
From a players point of view, whilst I would agree that the huey is the most "popular" image of Vietnam, the ability to play the more obscure (riverine, SF etc) would appeal.
One niave thought. Would/could someone be "transfered/promoted" to a different arm of the forces? Eg, start in SF, does so well that a transfer is made to riverine so they could use skills learnt by the ex SF soldier? The story could then develop further...