Hi Kadar,
I would perhaps experiment with my Stance to begin with, Is your Weight distribution on the Rear Leg to begin with?
If this is the case you may find it very difficult to counter a Shoot/Grapple this way, try a 50/50 Weight distribution and try to maintain you Centre of Gravity throughout all of you movements. You may be surprised by how much more Structure your Techniques will gain, when your opponant attempts a Shoot/Grapple think Jum-Sau, Gum-Sau, etc as one tactic.
Weight Distribution may also assist with the problem your experiencing with your Opponants Punches, you need to maintain mobility throughout all ranges. Regarding your Opponants Punches being Thrown and Returned very quickly, remember as a Wing Chun Practitioner yours should be doing exactly the same, "One Hand in, One Hand Out". Try stepping "Off-Side" to your Opponant, one hand will be less of a threat to you that way, giving you alittle less to deal with.
Something I would suggest however is considering what you wish to acheive from your Sparring Sessions? How long have you been training Wing Chun? How long has your Opponant been training what he or she trains? Do you wish to Master Wing Chun, or do you aspire to becoming a Mixed Martial Artist? This may all sound a little anal, but if you havent progressed too far through the Wing Chun System and your Opponant is very skilled/experienced in what he or she trains, the out come is pretty much preditermined before you begin. If you can throw your ego out of the window and just concentrate on developing YOUR skills, then great go for it!!
Good Training
Jin-Ka