We have a 3 year contract for software with a vendor. It has not worked well, and they have not offered us the support promised in that time. 

We told them we wanted to terminate the contract nearly 18 months ago, and the executive team had a conference call promising they would fix the issues and we wouldn't pay any more until they were all fixed. They gave us a bunch of free consulting
time which didn't end up happening because they said the product didn't do what we had been told. That process took 12 months, though they told us we had to start paying again after 3 months.

We have continued to wait for them to fix the issues, but it's now nearing the end of the contract, and we have gone back to say we would like another 3 month credit which they have refused. This basically is to make the product "most cost effective" which partially reflects functionality which doesn't exist nor likely ever will.

We are wondering on our legal options. One option for us is to tell them we now need to go back to the discussions of 18 months ago, and refund us the monies paid and the interest on those monies. They keep just saying they won't issue us the credits we had asked for.
The other is a complete refund of the product.

Anyone here had any experience with legal action internationally? Ideally USA specifically? 

I understand I will likely need to consult a lawyer at some stage, but I was keen to know if anyone had done this etc.

We believe we have proof of a fundamental breach of contract. We also believe we were mislead as to key functionality that doesn't work as was described.