Do you know why no one has metioned any wing chun solutions to the double leg takedown (or single leg for that matter)? Because that system really doesn't have a said defense against it. That system wasn't designed for that.
The fact of the matter is, not every system has the answer for every attack. So the solution is, to find a defense that works, regardless of where it comes from. Although I personally think that trying to confine one's defenses to a particular system is stupid and dangerous, I will give a pointer on it.
Simply try having a wrestler, or BJJ'er, or MMA'ist friend come at you over and over again. Try using different wing chun techniques, be it the chain punch, a knee, the YGKYM rooting and some attack, a step away and redirection, or whatever you want. Be pragmatic about it and see if it works.
Then, try the defense the way the guys that defend against it all the time do. In other words, try the defense that other wrestlers, BJJ'ers, or MMA'ists do.
See what works for YOU, and stick with it. Loyalty to your style or your teacher is all well and good; but only you are going be responsible for protecting yourself...your grand sifu isn't going to walk around with you to protect you. So my advice is to get as good at wing chun as you desire. Focus on it your whole life if you want, but try and understand the dynamics of it, and don't be afraid to acknowledge it's design limitations (in addition to it's strengths). Honesty is the best policy when talking about fighting. Over-confidence is a killer.
Long story short: just about every martial arts system is simply a quid pro quo method. You give up learning x skill to learn y skill. The reason is because most systems are based on a particular area that the founder was especially good at, thus the reason why the style became popular.
Good luck!
"I don't know if anyone is known with the art of "sitting on your couch" here, but in my eyes it is also to be a martial art.
It is the art of avoiding dangerous situations. It helps you to avoid a dangerous situation by not actually being there. So lets say there is a dangerous situation going on somewhere other than your couch. You are safely seated on your couch so you have in a nutshell "difused" the situation."