Hey there! A little bit of trivia about the name: I have been working at an internal project at SoundCloud called goku (not the web framework). It came the time where I needed to load test it and didn't find the available solutions to my taste. Hence, vegeta was born :)

Regarding the cluster mode, it's trivial to sync commands across machines but it's not trivial to sync the state that generates the reports. That's what cluster mode would do. Some sort of master-slave architecture scatter execution and gather and compute the aggregated reports.