After conquering a rebel land!

[EatYerGreens]

Now I think about it, I suspect you may be right.

As Byz, only three battles with rebels to date. Serbia, I remember seeing the execution choice screen but forget what I did with the prisoners. Definitely don't recall any V&V alert, whatever my choice was.
Crimea, 40 mounted serges retreated to castle. Fought my first non-autocalc castle assault since VI and killed every last one of them.
Khazar, castle fell in year two and I didn't attempt assault. No prisoner message that I recall other than 'confiscated lands'.

So it's either lack of opportunity or there really are no more rebel_mercy V&Vs in VI....

[Budwise]

I have gotten Mercyful before. You just have to ALWAYS do it.

Also, another topic but it was brought up here.

HAS ANYONE EVER HAD THE VIKINGS TURN CHRISTIAN? I never have.

[Bregil the Bowman]

HAS ANYONE EVER HAD THE VIKINGS TURN CHRISTIAN? I never have.

My first foray as the Saxons ended up with the Vikings in revolt, allowing me to take over Scandinavia and leaving the rebels in charge of Denmark. I cleaned up the remainder of their colonies on the Scottish coast and Man to destroy their fleets (which were preventing trade and the free movement of my armies).

Later on they re-emerged in Ulster with a couple of good stacks, but with Ulster being a good Catholic country their king immediately converted. After that they gave me no trouble whatsoever, didn't build any fleets and stayed as good neighbours until I chose to end the game by invading and destroying them.

Earlier in the game I had sent about a dozen bishops to convert the Viking homelands, but they resisted to the bitter end.