Though most of the detailed information on this will be within the different vampire classes, I will quickly cover the basics here. For starters, all vampires have the ability to shoot out and retract their fangs, much like a cat's claws. Most vampires have four fangs, but they can actually have anywhere from two to eight fangs. The four fangs that are in place of the four eyeteeth are called the major four, and the major four for lower class vampires look exactly like regular eyeteeth at first glance. The only difference is they are significantly sharper than typical eyeteeth, though their sharpness is inconspicuous. Lower class vampires will have either two or four fangs. Two fangs means the vampire has half of the major four, the top two to be precise. Four fangs means the vampire has all four of the major four.
Upper class vampires have six or eight fangs. This means they have all four of the major four as well as either the top half or all four of the smaller inner fangs known as the minor four. The minor four are shorter than the major four, and rest next to the major four on either side of the two front teeth on the top and bottom. When a vampire has half or all of the minor four, the major four that rest next to them will be a little longer than typical eyeteeth, thereby giving the vampire the appearance of having two or four small fangs. In other words, a vampire with six fangs will only have their upper major fangs a little longer, where as a vampire with eight fangs will have all of their major four a little longer.
So what about when their fangs are shot out? The size and shape of their fangs differs according their class, but one thing is consistent. Even the smallest of fangs will appear approximately two to three times longer than their retracted state when shot out. This is why the vampire's jaw is built to open extremely wide while at the same time looking completely natural. This is also why their fangs naturally flex in and out of retraction when a vampire speaks while their fangs are shot out, thereby causing the vampire to naturally hiss through their fangs.