It's unfortunate that they lied to you. And, unlike most cases we hear about here on FT, in this case, it sounds like the agent really did lie, if what you're telling us is accurate.

However, I'm not sure that you're due anything. The $50 you got may have been $50 more than you were due, especially considering that they paid for your expenses during the two days. Had you known that the vouchers were good for only $50, what would you have done at the time? Demand they fly the broken plane? Demand that the pilots work past their max duty time? If they cover your expenses, anything you get beyond that is simply goodwill on the airline's part, unless some statutory/regulatory mandatory compensation applies (such as for EU carriers or flights departing from the EU). Other than that, you're due nothing other than transport as ticketed on the next available flight, which it sounds like you got.