The vision center concept was developed some years back and I'd trace its beginnings to Richard Kranicki's interesting volume, Answers To A Pool Player's Prayers. Kranicki, who had a hand in his work from no less a person than hall of fame legend Willie Mosconi, noted that all types of parallax views hamper aiming when changing body positions vertically and horizontally above the cue ball and object balls.
One insight I've reaped from adding to Kranciki's excellent work-pick out your spot or edge on the object ball but do not stare it down as you lower your body into the stance. Take the object ball spot as an approximation while standing erect above the balls, Look away while you settle in to your stance then find the object ball again.
To have a fighting chance to pocket a lot of balls, since many players have a decent stroke but their aim is slightly off, you need to get your vision center over the line of the shot between cue ball and object ball. Vision center may be defined as "the spot on your face that needs to go over the cue stick so that a straight shot looks really straight".
In other words, if you are listening to the old dumb advice of "stick your dominant eye above the cue" and "get your chin over the cue" (two pieces of "advice", by the way, that create an inherent conflict) and you "see" a shot is straight in when it isn't, no wonder you have so much trouble with "straight shots".
Let me tell you how to discover your vision center, and you are likely to discover your aim system is pretty solid already.
You'll want a friend's help to do this right. Drop a cue directly on the midline of the table (butt end and tip end pointed at the two diamonds that mark the center of the short rails). Make very sure that the cue is lined up dead straight while standing erect above it, your chin above the cue.
Next, stand above the cue in your stance without touching or moving the cue. Get your head in the stance until you are ABSOLUTELY sure you can see the tip of the cue points to the middle diamond PERFECTLY dead on.
Now have a friend verify where your head is in relation to the cue. Most people do not see the line best when the chin or one eye is atop the cue. Most right-handers see the line best when the cue is beneath a spot on their cheek somewhere between their nose and right eye or just under the inside corner of their eye instead.
You can use a mirror with a piece of tape on it as a pretend center diamond to try to find your own center, but even there, you're likely to get a parallax view and a friend who drops a cue stick or ruler from your cue to in front of your face will be the most helpful.
As a side benefit, this drill and a good friend can help break other poor habits such as having your stroke arm bent under your torso at an angle or having a head tilted sideways.
Bear in mind, stance is built around a good concept of the line of aim of the shot. Often, rails and objects interfere with the regular stance as well. The main thing is no matter how you stand to get your vision center over the cue stick rather than having your head in different positions day-to-day. Having an inconsistent head position helps explain why you have good days and bad days at the table. I only have good days and better days in pool because I see the shots the same way every time.
Again, when the tip is pointed at the dot on the mirror or diamond on the table and you then bend down to see it as such in your full stance-dead square straight ahead-you know you are at "vision center" in your true, correct stance. Otherwise it's no good seeing a straight cue in the mirror--if it isn't straight and you have an optical illusion. Your shot making will have trouble for years to come.