Getting fired over recording

Can an outside company (a company that fixes your equipment) record a conversation during a troubleshooting conversation) without telling you and share the recording with your employer to get you fired simply because the owners are both friends? I'll explain in more detail.

I got let go a few days ago because I had said something about my employer to the vender who fixes our equipment. I was upset because my employer had no RMA to fix my equipment in over a month and all the other trucks were fine and have AC. Not mine. The faulty equipment was slowing me down and making me look bad. I was doing some of the largest wet-hosing routes, even the new guy I was training told me I have one of the hardest routes. The inside of the truck was 120F in southern CA because my truck had no AC. So maybe the handheld was overrating. I also never got my uniform even after 4 months of service while all the new guys had them.
Anyway, It turns out that the owner of the vendor who recorded me was friends with the owner of my employer and played the recording for him. I was unaware that I was being recorded as I was speking to the service representative. They don't tell you. I never said anything bad about this vender and I was polite to the service rep, but the girl played the recording the owner of her CO who is friends with the owner of my company.

I didn't think it was legal in the US to record someone unless the people were aware. But then to go and use that recording to get you fired? I have a recording I have just captured of this vender's phone system-message as proof that their message prompts DO Not warn customers first. (I am the customer) I also got one of the support tech guy at this company to admit that they don't usually warn their customers that they're being recorded. I have that recorded also. This vender is not a customer, it's the company who fixes the servers in our trucks.

As far as my work and what was going on. I deliver fuel, and we have handhelds. For some reason the hand held in my truck keeps dropping out. Every day it's something. The other day there's some kind of new problem. The pump blow a seal the other night and I'm stuck cleaning it. They don't even have a sink or soap in the yard for their employers to wash up at the end of the night and they've been in business for over 10 years. I've never worked for such a dysfunctional employer my life. I was also getting harassed by the new dispatch guy who's more like woman that has obviously never had any kind of power and it's gone to his head. I busted my tail for this company, all the customers liked me and my work history is perfect. Except for this incident. I could write a list of other issues I've dealt with. So the owner told me he has to let me go because I was badmouthing the company. Fine, understandable. But this other vender who recorded me seems in violation of the law.

Lesson learned, keep your mouth shut. However, I'm so happy to be out of this place.
In any case, can a vender (another business) who's not even a customer record you and then turn it in to your employer legally when the recording has nothing to do with them?