This is a pretty major thing. There is no other way about it.
It isn't an ethical thing it is illegal in the US.
And this wasn't your local HR rep, but the VP of HR.
So know that you bringing up this at your company almost assuredly means that you have to go. I mean even if it seemed that your company handled this as properly as they could you will never understand if you have been blacklisted from future advancement. The fact that the statement was made at such a large conference probably means that it has backing. Which means that they may fire the VP of HR but the 20 other higher management figures that stood there and clapped are still around.
So what do you do? Step one will be calling a lawyer. You will need someone that specializes in labor law or in whistleblowing. Step two is probably doing a complete inventory of your life at the company. Getting copies of performance reviews, copying over your email files, copying over current work - your lawyer will get you a list.
Then there is probably going to be some exploratory investigation you will need to do. It is quite possible that your lawyer will have a few people that he/she wants you to talk to. You may need to contact an HR rep and say something like, "I have a friend John that is interested in a position here. But he works at XYZ (competitor that you might have an "agreement" with) and after I heard Joe (VP of HR) speak I was wondering if he should even apply here. Can you confirm that XYZ is not our list of companies we agreed not to poach from?"
Let me tell you I have a good friend that went through this on a much smaller issue. It is not fun. You will be asked to do something to further your "allegations". That it was said at a conference is really good for you. But you will probably be asked to work the case a bit - I would expect it before making your decision.
My friend was "suspended" (fired). She was not paid for a long time. I would expect that this could happen too. It isn't about paying your lawyer - any lawyer will be paid by winning case not an hourly. It is about what are you going to live on while this gets sorted.
The only other option that you have - if you want to take action - is report your company anonymously. But let me tell you something. Almost anyone can be tracked and the other part of it is, action taken against your company is not likely unless they have received other complaints.
Note: Trying to handle this yourself is not a good idea. Just bringing up this could sound alarm bells everywhere. If the company thinks you are a threat they will belittle his statements as exaggerations, tell you it never happens, apologize, and then quickly try to move you out of the company for performance issues. This might not happen every time but it is a huge risk, with no reward. So if you are going to do it, do it right.
This is not a ethical dilemma. It is all about risk/reward. Very quickly you will know through your lawyer where your case stands. The lawyer will not want to go forward unless they almost know they will win. You on the other hand will get paid for being the whistleblower and having to leave the company. But this might not be a HUGE sum of money and your reputation in your industry may be tainted.