Vriska would have required more care then just getting the arm from equius after she blew her arm off. I mean, the explosion itself could have easily killed her. It didn't... But she also bled half to death. Leading to a very interesting headcanon that equius has some knowledge of medicine and first aid.

Equius’ mechanical implements are strangely robotic for Alternia’s biotechnology, but we know his bots are capable of having things such as blood pumping through them, so they still seem biological enough. If that alone doesn’t have enough weird troll stuff to keep Vriska from dying, do remember that he’s connecting functional limbs to people. The arm may be a tad easier, but with Tavros’ Legs, he would have to connect it to his spine and ensure they work properly. Forget ‘some knowledge of medicine and first aid’, Equius is very likely on par with a neurosurgeon.

We also now know Trolls have magic fleshy glory holes that can repair broken limbs so there’s also a possibility he just has a Medicalizer around.