The influence cost of integration makes integrating vassals a waste of time and your rarest resource. You may as well just release them as a subject and wait the 10 years until you can declare war.
I beg to differ, it depends on the situation really:
Since 1.4 protectorates usually start being vassals pretty fast and then you can integrate them 10 years later. Also you shouldn't forget that protectorates give you +0.5 influence every month, so you accumulate quite some influence before you integrate them, which should pay most of the cost of the integration after that, the pops are fully working and not unhappy.
That's the way I usually go.
Waging war to cede on the other hand will generate threat and make their pops unproductive for the first years, but is the fastest way to get the planet.
The third option, if you are not individualist and their ethics sucks: You can Liberate them which will give them your ethics and usually they'll love you enough to join instantly as a vassal. 10 years later when you integrate them you'll have pops that are shifted way more towards your own ethics as the homeworld gains huge boni for it and they won't be angry (unhappy) at you.