What was Mithos's vow(s) to the Summon Spirits?
It was a promise that he would restore the Great Seed to the world so that it could grow again, thereby restoring mana to the land. Instead, Mithos took it to Derris-Kharlan in order to revive and protect his dead sister, thereby breaking his pact with the Summon Spirits.

The pact that Mithos made with Volt was not one of a promise, Mithos's pact with Volt was to use his power. Because of how Mithos acted after gaining the use of Volt's power via the pact formed, Volt became angry at the misuse of his power. This is similar to how Lloyd's group uses Volt's power to energize the Rheairds. 

The other Summon Spirits are able to communicate with Mithos with forming the pact, and didn't accept a "use my power to do whatever you want" pact - he had to be more specific. The only reason Lloyd's party was able to communicate with Volt was the addition of Raine.

Since Mithos technically didn't do anything but use Volt, he didn't break a vow.