You've gotten some good answers already.. you really do need to play with it.
A good powerful vang can take the place of the traveller, ie all the leech tension can be created by the vang if it has enough purchase and if the boom is stiff enough to transfer all that force through to the clew.
Consider the standard, 'traveler down, sheet on' to depower in a breeze.. the traveler adjustment adjusts the angle of attack of the sail, then the sheet provides the required leech tension. Once you get to the end of the traveller, as Mark said above the downward force available from the sheet goes away and the vang is required to limit boom lift, keep the boom under control and prevent a 'goose wing gibe', a dangerous event prevented by any vang, even a fixed strop.
With a powerful vang, upwind, when you 'run out of traveler' easing the sheet now adjusts your angle of attack and the vang prevents any further boom lift. This is sometimes referred to as 'vang sheeting', it mimics having an 'infinitely wide traveler' and is a common small racer/dinghy technique in a breeze. However if you're doing this it's imperative to ease the vang prior to rounding a windward mark.... bearing off can generate such forces as to literally tear the vang fittings out of the mast/deck/boom etc (.... don't ask......)
So sailing a dinghy
with no traveler, having a good vang and getting used to using it should be a great advantage.