Traveler and sheet are your vang - yes. The traveler is long enough (and you don't sheet out so far as on a monohull) that you don't need a vang.

If you are going with a square top, upgrading to a boom would be worthwhile. The boomless setups were driven largely from safety - removing one hard thing that can significantly hit you. In order for them to control sail shape well, they need to pull a significant angle to toward the mast to control the draft of the sail. However, with the square top, you will need much more leech tension to bring the square top into play. Having a boom and good geometry between the traveler and the boom/sail connection point is important.

Make sure your sailmaker knows what the foot dimension of the sail needs to be for the boom geometry to be correct. You need the sheet to angle off the back of the boat enough so that it pushes the boom forward with enough pressure to induce mast rotation but not so far that it unnecessarily loads the boom and gives up leach pressure. You will also be able to simplify your mast rotation controls since the boom will help take care of this now. Also consider connecting the sheet directly to the sail clew with a strop around the boom. This will help reduce the structural requirements of the boom since the majority of the sheet loads will now go straight to the sail and the boom just carries the inward rotational forces to the mast.

If you can get a chance, look at an F18 rigged and take some measurements from the sheet/boom geometry. That should get you started.