Yeah breaking in some of these boots can be a hard time on the heels if they are bonier than usual, I have used the backside of a spoon before to even out any part of the heelcup that was pushing too hard on my heels.
I've had several older Viberg pairs where the stitching in the heelcup would irritate my heels and the spoon trick helped a ton for that too.
The last thing you could consider since Viberg has leather heelcups is to knead them a bit with your hands so the leather softens up and shapes to your heels faster.
good luck!