I would imagine that HPH is one of Vietnam's smaller airports, but Mai Linh will probably be represented there, but that still doesn't mean you won't be scammed. I was scammed by Mai Linh in Haiphong a few years ago after arriving by train from Hanoi enroute to the Cat Ba piers.

You seem to think that Vietnam is still rather backward, but nothing could be more wrong. You will have no problems there getting around without knowing Vietnamese. Haiphong is the major port city in N. Vietnam, and not terribly tourist friendly, but still manageable.

Sound like the bride is worldly wise, and fairly westernized and upper class by your description. I can't advise as to a gift, but I can tell you a little about color. I would avoid the color white (the color used for funerals in VN). Red is the lucky color, but often the color of the bride's dress, although not exclusively. I think you'd be safe with something in light blue, green or tan.

Regarding customs, I would not greet anyone by hugging them as we might do at home, but this website outlines VN customs quite well.

What to do with extra time in the area - well, most visitors to Haiphong prefer to cruise Halong Bay. I know the agent who represents several 1 or 2 night cruises on the Bay - I recommend the more expensive ones.