tunnel vision blinded me so much I could not even get a visual of the first shooter I saw. How can this be prevented?
Prevented? Nah.
Controlled? Yes.
When we get confronted with a terrible situation, nature has instilled survival mechanisms in us that kick in automatically, one of which is the "tunnel vision" you referred to and experienced.
You are NOT going to prevent your body from doing what nature designed it to do, BUT, you don't need to in order to choose and execute a course of action, and you DID choose a course of action. A sound one imo.
You were in a secure area.
You didn't initially expose yourself to be mistaken for a combatant.
Solid imo. Can we do things better? Yea, but hindsight blah, blah, blah.
It has been my experience that in order to perform effectively in stressful situations you must first recognize and accept the fact that your body is going to do things that you can not control. Practice is the other component, and outside of military and LEO training, most folks just don't get the practice.
Adrenaline spikes, emotion, increased heart rate, breathing, sweating, fixed / tunnel vision. All of these things will take place, but it doesn't matter. People will often make the mistake of recognizing these symptoms as they occur and actually becoming increasingly excited because they REALIZE that they are excited. In a sense they get scared because they realize they are experiencing fear, which is just another survival mechanism as is a raised pulse.
When the body does it's thing, let it. Stay focused on what you need to be focused on, and make a conscious effort to remain calm and do things SMOOTHLY and deliberately because in a heightened state, you will tend to move and do things FASTER than what you perceive. Eventually your body will settle back down and you will grow more comfortable with your new stressful environment. This may take seconds, it may take minutes, but it WILL happen if you are a strong willed individual.
Rather than eliminate stress, which you can never do, EXPECT it, ACCEPT it, and stay focused and you will do alright. As you did in that encounter imo.