Oh my god, I lived through this exact nightmare last summer. Our photographer just did not show up on our wedding day. No call, no text, nothing. We found out later she had double-booked and chose the higher-paying event. I am still furious thinking about it but I did get a resolution, so let me share what worked.
First thing I did was gather every piece of evidence: the signed contract, all email and text communications, proof of payments made, and documentation of the replacement photographer we had to hire at the last minute (which cost us 3x the original price because it was same-day). I also had guests who were willing to provide statements about the impact on our event.
Also leave honest reviews everywhere. It will not get you your money back, but it will warn other couples. And check if the vendor has insurance, as many event vendors carry professional liability insurance that covers exactly this type of claim.