The braising liquid can be served as is, especially if it's already reduced enough to have reached a 'sauce' consistency. Otherwise, as mentioned you can bind the liquid either with a beurre manié, which is simply flour blended with softened butter, or with a cornstarch blended with cold water to make a slurry. The beurre manié will lighten the color of the liquid and render it opaque, while the cornstarch slurry will thicken it without changing its color or transparency. If you've used the marinade vegetables as a soffritto, then you can simply blend the liquid and vegetables together, or pass them through the finest mesh of a food mill, which will emulsify and thicken it 'naturally'.