true. wonder if there's something like the average of the 2 best bids that would mitigate this problem. coupled with anti-corruption enforcement...

Indeed, you would need carrot and stick. The point the witness made was that under the current system you could not profit without collusion. (Edit to add: and that means that all honest people end up driven out of the construction industry). At least under a Vickrey system an honest person can make a profit hypothetically proportional to the social benefit provided.

The flip side is you would have to tell people that while you're going to increase what you pay them your tolerance for collusion (and it is essentially tolerated today) will go down, and mean it.

They are vulnerable to collusion in the sense that if all partys collude to decide who is going to win, then the winner bids X and the losers never put in a bid, so the pre-determined winner pays $0. In that sense, literally every auction is vulnerable to collusion. The only net downside to the Vickrey auction is that the winner still wins even if there is a defector, they would just pay more. This is in contrast to say a normal first-price auction where the winner values at $100 and the losers value at $99, so they collude to have the winner bid $1 and the colluders bid $0, but then there is a incentive for the colluders to defect and pay $2 to get the thing they value at $99. So, vulnerable to collusion in this sense is more that it is vulnerable to collusion with no little to no risk of defection. However, if there are non-colluders, then the best you get is the highest bid of the non-colluders.