Are the creatures in I Am Legend zombies, vampires, zompyres, or something else?
In the book, they are much more explicitly vampires. Even in the Will Smith movie, the creatures are vampiric (and referred to as such, I think) - they hide from the sun, which burns them, they drink blood, and they are transformed from normal humans.
In the book, there's a difference between vampires - some are dead vampires and some are living. The dead are mindless monsters, the living can be civil and have their own society. This is not reflected in the Will Smith movie.
They are NOT traditional vampires, if that term really means anything anymore (I'm looking at you, Buffy, Twilight, and Harry Dresden), but they absolutely are NOT zombies.
Zombies are not intelligent, do not have any form of social interaction, do not fear the sun, and can't become human again. All of this runs counter to the movie.
In the earliest stories of the undead, they were purely creatures of magic. There aren't many variants either, almost all of them are what you and I would recognize as vampires. These are creatures that drink blood, kill and convert the living to what they are, can't abide the light of day, and can't easily be destroyed. None of their traits or abilities need to be explained by physics.
In the modern movie, they are some sort of mutant or persons afflicted with a chronic condition. Though they do not have any obvious magical powers, they seem to have much more energy than can be accounted for... what are they feeding on? Stronger and faster than the best athletes... but they aren't eating Michael Phelps' 10,000 calorie diets (that we can see). If they really do eat people... why haven't they all starved? If they are just feeding off what food is left in grocery stores and the like, why aren't they fighting each other over it (or if peaceful, cooperating with Will Smith)?
Other movies make vampires/zombies more clearly magical (remake of Dawn of the Dead). Yet others don't care, they exist solely as a vehicle for horror and gore.
Everything considered, I Am Legend creates its own new creature that merely borrow from vampires.
In no way are they zombies mostly because zombies are not intelligent nor do they fear light. We see intelligence when they stalk towards nevilles house and attack him there. Even he said that they could follow him. Also they are bright enough to know how to hunt and know how to defend themselves. They are not quite vampires either. For one they do not drink blood, going back to the deer they had caught only its head was remaining and all the meat was gone. They also have the possibility of turning back into humans which is idiotic to even believe unless you are watching twilight. They are simply infected. The virus caught on because they believed they had a cure for cancer. Which these mutated viruses became airborne. It was a mix of different types of terrible diseases. They are a new creature all in their own.
I use closed captions while watching movies, and the captions actually referred to the creatures as "hermaphrodites"...in my mind that is just politically incorrect, and it always had me wondering as to why the captioning company would refer to them as something like that unless they got that term from the moviemakers themselves. Turn on your captioning and the part where the deer runs into the dark building, you will see in parenthesis (hermaphrodites panting) or something to that effect.
The movie I Am Legend does talk about these strange creatures. But, they die easily when shot by a gun, so that provides with another question to answer: "Vampires don't die easily, right?". Zombies, however, die easily. So, are they zombies or vampires? I believe they are zombies. Because in the movie Will Smith and his family were driving to safety and one of the creatures went through the window and bit the wife. Normally, if it was a vampire the blood would have been gone immediately, but it took some time for her to die.
I Am Legend is based on the book I Am Legend by Richard Matheson. It's a 1950s era book when the zombies as we know them today didn't even exist. In the book they are basically vampires, he uses mirrors, crosses, garlic, stakes and the like to ward them off. He researches their symptoms as if they are vampires.
In the movie, the creatures are somewhat like vampires but we see no direct evidence that they drink blood although they seem to be attracted to blood (which is used as a lure) and perhaps they do explicitly drink blood since instead of "dark seeker" in the original script a term is used for them that involves the root "hemo." (Hemocyte is the term that does not mean much afaik besides the root.)
Importantly, in the original book by Matheson, they are exactly "vampires" -- that is the central idea of the story, that what we used to call vampires were in fact victims of a bacteria that caused them to behave just like legendary vampires, minus the supernatural aspects, like they could not transform into bats, etc. The closest thing to a supernatural characteristic is that people who die from the plague reanimate, driven by the bacteria and such reanimates behave differently than people who do not dies from the illness.