Well first, when Angel was put into the Box he was sealed shut completely so no water got in at all. The reason he was all disoriented was b/c of the lack of blood that he hadn't eaten in a long time.
Second, I think that the difference between vampires and zombies needs to be determined. I think that the zombies are really the undead people but they were raised by forms of magic. Vampires on the other hand are living things. They are a demon that has taken the form of a human. Like it is explained in the WttH, the last demon to leave this Demension fed off a human, mixed their blood. They now walk the earth half human, half demon. Like Anya explains in Graduation Day, no one has seen a true blooded demon. Everything that Buffy has fought has been some form of half-breed. Through this explanation I think it can be said that Vampires do breathe, but the act of suffocating will not kill them. The only things that can kill a vampire are sunlight, holy water, stake to the heart, beheading, fire, etc. Anybody have any thoughts on that?