With so little time, I agree that your best bet is to book with Hanoikids and Saigonhotpot (just google for email addresses). I do not agree that you need to tip at all. Both times that I've taken tours with Hanoikids, they have refused all monetary gifts from others in my party. Tipping is not the custom in SE Asia and these students are volunteers who exchange their knowledge for English practice!
Please return to Vietnam without a cruise sometime - this is an amazing country, and one sees very little from the deck of a ship.