One could imagine doing some kind of differential testing of another key-value store against vard to ensure that for a random but large number of requests, they return the same value. I'm not sure that doing that would gain you anything over using Jepsen [1], though. One could also imagine implementing another key-value store in Verdi and proving it equivalent to vard; reasons to do this might include using better data structures or serialization mechanisms, or a different consensus protocol. I'm not sure that this would require significantly less effort than proving this new key-value store (usrd? whittend? :)) correct from scratch, but I'd love to see someone do this!
[1] https://github.com/aphyr/jepsen