In Illustrator, how do I move all vectors in 1 layer to top-most layer-tier and discard the sublayer containers and retain the original layer name?
Basically, bring all sublayers and contents to the top in 1 layer.
As far as I'm aware there's no one solution which works for everything all the time.
Copy/Paste to a new layer is a good solution.
There are also some little used Layer Panel menu items that may help.
Note that no artwork is selected in this animation. It's all merely moving layers and layer structure.
Three layers, with nested sublayers on a couple of the layers.
How useful this is depends a great deal upon the current structure of any layers.
Depending upon how deep nested sublayers go, or how prolific they may be, it may take using the commands a few times on different sets of highlighted layers/sublayers.
Although I have all the layers/sublayers expanded for the animation, these commands work on collapsed layers. So, you don't have to expand layers, just highlight them and and choose the menu items.
Unlock all layers you want to move from.
Select All (Ctrl+A)
Click on topmost layer, and paste-in-place* (Ctrl+F)
Now select extra layers and drag them to the trashcan.
* Make sure you have Paste Remembers Layers turned off. Otherwise it'll recreate the sublayers etc. (Click on the dropdown in the layers panel to turn it off.)