Reasonably competent Wing Chun practictioners can be quite a challenge...I'm guessing the reason you found few holes in his defense was because of his emphasis on dominating the centerline.
Not really aikido stuff that I have tried against Wing Chun guys: Round kicks to the legs (but you better have your round kick down cold). Side kicks, from abdomen down. LOTS of lateral movement. This stuff worked OK for me back then (when I was younger), but it sure wasn't guaranteed. I wasn't (and am not) much of a grappler, so I seldom tried any wrassling takedowns.
I'm sure you can find a way to apply aikido principles when practicing with a Wing Chun partner. Off the top of my head, don't let him dominate the centerline so easily; make him work for it. In other words, you make damn sure you occupy your own centerline. Secondly, when those chain punches start coming at you fast and furious, the WORST thing you can do is back up in a straight line. Maybe you can tenkan your way into a nice kokyo ho. Then again, maybe not. Hopefully, the more seasoned aikidoka here can chime in...
At any rate, keep working with this guy; I'm sure you'll pick up a few things. Make sure you let us know.