Q: How did Tarantulas have knowledge of the Vok when no one else had ever heard of them?

A: Tarantulas and the Tripredacus Council were members of the Cybertronian Empire who reassimilated back into life on Cybertron after they were reformatted after a run in with Unicron, and the Hub being destroyed by the Autobot/Decepticons alliance. Double-whammy.

However, during his days as a loyal CE troop member, the scout ship he was assigned to (to find planets to be techno-formed into the Hub) discovered a world that also had Vok installations on its surface to test the indigenous life forms. It too was seeded with raw Energon.

They made to take over the planet, but the Vok were notified by one of their traps. The planets' moon opened up and destroyed the planet.

Thus why Tarantulas was so keen to get off Earth when it became evident that Earth was to be destroyed - he'd seen it all before.

So he knew of the aliens, hated them, but didn't find out their names until he met Tigerhawk.

Simple