When comparing Wing Chun & Jeet Kune Do it is important to see the reasons that Bruce Lee thought necessary to try diffrent techniques that led the way from Wing Chun to Jun Fan & eventually Jeet Kune Do as he left it.
 
The technical reason comes from the modified version he learnt, in the modified version a practitioner is taught to face an opponent centre line to centre line, in the modified Wing Chun the opponents centre line is the main focus for attack & defense of your own.
 
Traditional Wing Chun also uses the centre line principle but in a different way, you dont face the opponent centre line to centre line, Traditional Wing Chun use's footwork to manouver and keep an opponent in an area instead of on a line, techniques are applied at an angle but still in the centre line area you are then able to cut off and control an opponent more effectively.
 
This is the principle that I think was what led Bruce Lee on the road to perfection, which he endeavored to achieve.