Off the Record reference check ruined job opportunity/former employer jeopardizing job opportunities

I need to keep this as anon as possible but I was let go from my previous job in IB. HR has been really cool with me in terms of helping me get a new job. They’ve allowed me to write that I’m still present at the company (didn’t do this), and extend my employment dates by including my severance package. Overall they were really cool with it.
My former VP has been helpful as well, but unfortunately other members at the company were not so helpful. Basically I was let go during a layoff and I was fucked over. They knew they fucked up it was a calculated layoff as they realized early in my tenure they didn’t need another analyst so they documented me for poor performance to fire me with cause. This is why they’ve helped me out with the dates of my employment. I signed off on my severance because I didn’t want to hurt my reputation in the space by suing since it’s a small industry but I was just told that my former employer fucked me over in an under the table reference.
HR confirmed with me that they wouldn’t tell them “why I was let go” or any details of my employment. I was told by the company I was recruiting for that they told them details, and despite performing well in the process they decided to go with someone else. Them choosing to contact my company without my consent is another matter, but I am happy because they told me because I never would’ve known. what can I do about my former company here?