For me it largely depends on how far along the game is, who my neighbours are and how they feel about me, my military strength, and the direction I want to take my empire.
Generally:
-If I do not have a very powerful military, I am more likely to vassalize so that I can conserve units, and to reduce the chance of someone else attacking me while I am busy. ALSO, I find that if I do not vassalize at the quickest opportunity, and my military isn;t overwhelming, the Civ I am attacking will often become the vassal of another Civ, who will then also be at war with me. I hate this when resources are already stretched thin.
-If I have powerful neighbours that are on my continent, and especially if they aren;t happy with me, I'm more likely to vassalize, to prevent the scenario I listed above, and to conserve units to prevent attack in general.
-If it is late in the game, I have to consider if there are one or more Civs that need crushed to prevent them from achieving a space or culture victory. If there are more than one, I will vassalize the Civ I am currently warring once they are knocked out of the running, so that I can sooner get to other Civs that may need attacked.
-If I have an overwhelming military and/or there are no dangerous Civs nearby, and I have no other Civs I need to attack, I'll simply crush any Civ I am attacking completely. No need keeping them around. Obviously I'll also simply crush any Civ I'm at war with before I have the tech to vassalize.
Anyway, there are a few other considerations too, which arise on a case-by-case basis. I just consider where I need to go with my empire, and what is best to achieve that. These are just my personal tastes, I don;t know what considerations others make.