Hi ! I'm a newbie here so I read and don't write but...
"Strong vertical acceleration means a strong CHANGE in the vertical component of the velocity. It doesn't necessarily mean anything beyond that."
Absolutely! You've got it right It is a simple statement from a frenchman with a scientific background. We do not talk of "deceleration" that is a word barely used in sciences only in more standard french. May I add the BEA report is very careful. The sentence is noted twice:
p.40 under the header "synthesis of an eye -visual- examination". To a frenchman this too is very clear it means this is what we see (with all the limitations it implies to a frenchman at least) nothing else.
Then it says "the plane has probably (vraisemblablement)) hit the surface of water in flight line ("with" but the word is missing here) a strong vertical acceleration"
p.72 Where it has to be linked with the previous sentence ("the identified elements are coming from all parts -the whole?- of the plane") and it says "their eye examination (again!) show that the plane was not destroyed in flight -because pieces coming from the whole plane dislay the same pattern of mishapenness so they might have been still in the plane when it crashed- it looks like it hit the water surface in flight line -roughtly flat attitude- with a strong vertical acceleration.
..italics +underscore= my comments.
Hope this helps our non/ french speaking and not familiar with the french culture friends here.