Can someone explain two things regarding pressing claims and the aftermath of wars?

So two quick questions, once I was attacking France and an option popped up: Press all claims. This was great because I could go after all 5 territories without having to deal with the 10 year truce. When does that option pop up?

My second question, is that currently I have the title of King. I could create an Empire, but because I'm stingy, I'm not spending money on a title right now. But if I press a vassal's claim for a kingdom...how will that pan out in the aftermath of the war? Will he still be a vassal even though he's an "equal" (a king)?

Or would you guys not recommend pressing a claim for an entire kingdom/empire, becuase even if they remain loyal to you, they will be too powerful for their own good.