sparring vs chi sau

[Si-Je]

Distance wise your correct. But unless someone has a weapon or is trying to kick you then at a far distance their not a threat to you yet.
Someone who's 5 feet from me isn't able to strike, grapple, or kick (unless their legs are really long! lol!) me. WC starts when the opponent is close enough to you to do some damage to you.
Chi Sau starts when you make contact with said opponent. Whether they punch at you and you "block" or re-direct as WC does, that's where sensitivity/chi sau starts. You keep that arm contact with the opponent. Pressure them, move forward into them, keep them from regaining their balance, or composure enough to try to strike again.
You really need chi sau to elevate your sparing to the next level. Otherwise all your doing is exchanging blows. You use chi sau to move with the opponent, feel their intention and deflect their next strikes. keep they pressured and off balance so they cannot collect a well formulated attack.

 

[brothershaw]

Many styles preferred distance is farther than wing chun, a wing chun person has to a) learn how to close that distance while being attacked in a very aggressive and dynamic way and maintain their prefered distance while being attacked thats where sparring helps
grapplers practice the shoot to bridge the gap , what does wing chun practice?

 

[Si-Je]

Chum Kui to bridge the gap.
Form and application.

All depends on the energy your opponent gives you. If they kick, garn sau, heel kick, etc.
If they shoot in to grapple you have their entire body, weight, and force to re-direct. Use leg sensivitity, etc. There's dozens of variables that you learn to flow and re-direct the opponent based on the energy they exert towards you. This training starts early in Wing Chun. Day one.