As the core of veganism is consent, there are several elements to consider:
- Consent of the substance’s sentient source: The coffee bean seems more than willing to give this up… almost too willing. I don’t think there’s a direct issue here, but definitely warrants further consideration when other parties are involved.
- Consent of the substance’s consumers: neither Mordecai nor Rigby seem to want what’s going on. While Rigby signed the contract, and could be held to its terms under ideal circumstances, it’s a rather predatory situation since it’s presented in a language he doesn’t understand. Mordecai didn’t sign anything, did not consent even on paper, and is a complete victim here.
- While mostly related to point #2, contracts are often wielded as a tool of the powerful against the weak, and in an inequitable exchange ends up being little more than a justification to continue a coercive arrangement one party would back out of if given the choice.
So I’d say while the events depicted are not vegan, but the coffee itself as a substance is most likely vegan. It’s a little hard to say for certain without more context tho, for instance if the bean is also coerced to act this way by outside forces.
Back in the day they would also be angry though, these men at least seem polite enough to leave a thank-you card