Huh, I don't remember that ever being said. Is that more book stuff?
The idea that vulcan last names are hard to say is kinda silly, they all have first names that are incredibly easy to pronounce but the last name is some kind of tongue twister? And it would have to be true for all vulcans because not a single one ever introduced him or herself with their full name.
Personally I think vulcans only have one name and Amanda was just kidding, she probably planned for Kirk to call Spock "Mr. Grmpflogsymprifgrm" just for shits and giggles with him turning to her and saying "Again, mother?"
Or maybe it's sort of a tradition. The last name not being meant to call someone but to put on official documents only.