This worked for me, although not retroactively.
The version of the game's .exe was already correct, but reinstalling GOG Galaxy actually fixed it. I redid 'Mystery of the Byways Murders" after the re-installation of GOG Galaxy and the "Fearless Vampire Slayer" achievement unlocked this time.
Just in case someone doesn't find this on the forums:
https://www.reddit.com/r/witcher/comments/3kizsj/possible_fix_for_goggalaxy_achievement_problems/
This guy may have found a way to fix achievements
Just in case someone doesn't find this on the forums:
<reddit link>
This guy may have found a way to fix achievements
Sadly the new method does not work or works only partially. I am running the latest version with GoG overlay, after the dude's method, I managed to unlock Card Collector but I had to redo the last gwent match to get the card. Other achievements (such as Mutant) do not retroactively get fixed