The weight of vegtan leather you should use for a sling seat depends on various factors, including the desired durability, comfort, and aesthetic appeal. For sling seats, it is generally recommended to use a medium to heavyweight leather that can withstand the weight and stress of sitting while providing sufficient support. A weight range of 3 to 5 ounces (1.2 to 2.0 mm) could be suitable for a sling seat, as it strikes a balance between strength and flexibility. Thicker leather may offer more durability and rigidity, while thinner leather provides greater flexibility and a lighter feel. Consider the specific requirements of your sling seat, such as the intended usage, user weight, and desired comfort level, to determine the most appropriate weight of vegtan leather for your project. Additionally, consulting with leather suppliers or experienced craftsmen can provide valuable insights and recommendations based on their expertise.