A little help for a beginner.

Starting with CK3 and have watched a bunch of YouTubers but a few things I need pointers on.

How do i know what happened? I was fabricating a claim against a country when it was now someone else's. How do I found out why it changed. Wasn't obvious it was attacked. So I assume some marriage or pressure to become a vassal. But how do I go and see why it happened?

What do you do if you lose a war and become a vassal. I lost, and was just an earl of 2 areas. I lost and became a vassal. If the area attacked was not mine but a vassal of mine would I still stay independent for the remaining area but that vassal moves over? Or is it always all or nothing? And if I am now a vassal how should my play style change. Try and become the best vassal and eventually hope something flukes in my favour if my liege dies?