Minecraft wiki entry for Combat, associated text: Vex summoning. Summons 3 small mobs, which can pass through blocks. They die within 2 minutes and are limited by a binding radius. When fighting in a room with evokers, keep an eye out for vexes, especially their red glow when charging. They are weak but deal large damage due to the fact that multiple summons can happen.
Fang attack, explained better in the main article. It does 6 of true damage (ignores the player's armor, potion effects and enchantments), it can hit multiple targets and it cannot damage other illagers. Fangs can be avoided if the player is constantly moving or jumping. Simply kill the evoker as soon as possible after killing the vindicators, especially if it attempts a spell. Evokers will use their fangs both offensively and defensively. When used offensively, they spring up in a line toward the player. When used defensively, the fangs form a circle around the evoker. A good strategy is attacking at a distance with a bow, then move in with a sword.