When you talk to Vannaka he greats you with " 'Ello and what are you after then?", after which you get four options. The first being: "I need another assignment". Second: "Do you have anything for trade?". Third: "About the task system...". And the last one is a cancel conversation option.
When you click the first option he assigns you with a task or if you already have a task he tells you you're still hunting something and you should come back once you've finished your task. He also lets you know what task you are still on. If you don't already have a task and you ask for one he tells you that you're doing great and then he lets you know what your new task is.
If you click the second option you ask him if he has anything for trade and he says he has a wide selection of Slayer equipment. The slayer shop interface then shows up. You can also access this by right clicking Vannaka and selecting Trade. His shop contains a Slayer Gem, Slayer's Staff and a Spiny Helmet.
When you click the third option and ask him to tell you some more about the Task System you get told that there isn't much information on it yet and you should come back later.
The Rewards interface (appears when right clicking Vannaka) isn't in place yet.