History of Vex Card Game

Overview

The Vex Card Game is a gambling card game played by Zephyrus and his crew of the Tharsis Reformation in the Tharsis Bazaar.

Quests

In Renegades you "Ante up with Zephyrus in the Tharsis Cantina" (Step 21), then place banners for the game throughout the Bazaar (Step 22). Speaking to Zephyrus afterwards (Step 23) shows he now owes you a favor. I found no source describing the rules.