I am not sure that ''needed'' is the right way but vangs and travellers go a long way toward helping to control twist in a mainsail. It makes a big difference in how your boat behaves in higher winds and as such adds to the saftey of your boat.
It''s hard to install a traveller, since you cannot just screw it into the fiberglass.
It is not hard to install a boom vang, since both ends are attached to aluminum spars. All you need is a rivet gun or tapping screws and knowledge of what you should do about "dissimilar metals" (search the articles).
Many sailboats have travellers so short that they are nearly worthless. Mine is only 2 feet across, on a 34-foot boat. So, I use the vang to pull down the boom, even close-hauled, and the sheet to control the angle of the sail wrt the wind.
The only thing I use my little traveller for is to pull in the boom that last 12 inches to the centerline in light air.
So, if you''re really concerned about this, consider installing a vang with a cam cleat. Don''t worry about the traveller.
One comment about the boom vang:
Consider installing a "rigid" boom vang. If your boat has a topping lift you won''t need it any more, and if you don''t have a topping lift the rigid vang will hold the boom up when you are lowering the mainsail--very convenient!! I have owned smaller boats in the past that did not have a topping lift (or a rigid vang) and it was always a pain when lowering the main sail.