depending on the type elbow used you response will vary. having said that, an elbow must be used at a very close range & unless it is coming straight up at a vertical angle it will usually take a rounded path.
seeing as though you are only sparring you can not use probably the best technique available, a straight punch. this when used in reality will always beat the elbow to the punch( pardon the pun)
i have also tested this against an elbow coming flat out( only in training because in reality i have found that by the time somebody gets to that range they are already being overwhelmed by my attacks rather than being able to use to much of their own) but a well placed tan punch combo can easily stop a full force elbow. it's the movement that sheers foward & can easily disipate the force of an elbow moving round.
now a good wing chun or thai elbow that cuts foward & down is alot harder to deal with & once exercuted correctly & in the correct range you are not looking good, i have developed my way of dealing with this but again the straight punch is always my prefered option.
as someone also said changing the angle is a good idea, however if you have time to do this then you must have had time to do something more economical( ie the straight punch)
i know you are only sparring & this is one reason sparring can develop bad habits, you can't use wing chun as the brutal art it can be. also in reality you have let the man get to close to start or you have made a mistake & either way you are looking like you may be in trouble.
sorry if i was no help but glad to be if i was, these are my own ways & will differ to others including yourself. train hard & learn to apply the principles of wing chun & i'm sure you find the answers you seek.
vts
[disclaimer- i am about to be rude, antagonistic & terribly offensive- but i love ya's all]