To see all that Vietnam has to offer, a cross-country itinerary is definitely the way to go. Although there are plenty of things to do in Ho Chi Minh City, it would be a real shame to limit your scope to just this metropolitan area. That is unless you have a limited time. If you are using Vietnam as a stop-over destination and have just two to three days, staying in the capital or a city close to international airports is the right thing to do.