Eye vector is unfortunately a commonly ignored facet of many self-defense programs. Eye vector like many other topics among the spectrum of defensive tactics & techniques depends on many variables, but there are some general guidelines. Where you direct your eyes depend on many factors including number of possible assailants, distance between you and the attacker,whether your attacker(s) is armed as well as your environment. In an unarmed one on one conflict, I generally recommend looking at the center of the opponents chest at shoulder level. Eye vector refers to the direction/area & intensity/magnitude of your gaze.Look at the center of chest area and look "through"your opponent.This essentially means not focusing your eyes on a pinpoint spot on the surface of your opponents chest, but rather trying to see the whole opponent by keeping the eyes in a somewhat unfocused state.
Most people will telegraph their hand techniques by some slight shoulder movement or some movement and/or tensing of the upper body or torso. Hands techniques are the quicker than leg kicks & likely to be directed toward your head & face,so they deserve a bit more priority over leg techs. If you maintain the proper eye vector, you should still be able to pick up their kicks just fine. Looking at the waist or legs will likely get you punched in the face or head by not allowing you enough reaction time due to a bad visual picture. Looking at the eyes is fine at a distance & during attempted de-escalation/negotiation phase to try to read opponents intent or emotional state since most people will show aggression or frustration on their face and you can use this along with general body language & positioning to anticipate a physical attack or escalating aggression.
If there are multiple opponents, you will use a scanning type of eye movement and the degree of attention on one assailant or the other(s) will depend on positioning and distance from you.Try to identify the dominant figure(usually closet to you, most verbal) as he will be directing overall group action & intentions.Try to stack opponents by subtle movements while paying close attention for any movement of anyone trying to get to your blindside and make sure no one gets out of your peripheral vision. If they do , you need to act by moving or punching a hole out of there with a pre-emptive strike.
If you are confronted by an armed assailant(s), disarms are a last resort, but may be necessary.The same above rules apply except that you will want to at times look at the weapon/weapon hand itself(usually only a glance). If you have to do a disarm your gaze will go from the weapon to his center(to gauge body position & possible other targets). If you are in a grappling situation struggling for weapon with a single attacker, then you'll be primarily focused on the weapon/weapon hand, but also have at least some visual awareness of surroundings and overall positioning just as you would in any situation.