It depends on what you want from them. A vassal is kind of a 'extension' of your empire: you join each other in wars, they cannot expand or conduct indipendent diplomacy and they boost your naval strenght. Tributaries are mostly free, they can expand and do diplomacy but need to pay you a quarter of their income. You may want to vassallize close empires to act as buffer states, quick reinforcements in war and to prevent them from expanding on the systems you want, and make a tributary of farther ones for easy income.