Gatekeeper won't let LibreOffice open .csv files
I had the same problem and finally managed to fix it.
Simply adding 'Libreoffice Vanilla' by doing a right click on a csv file and choosing that app did not remember this for all csv files, as you state.

I was able to configure it by going to the info of a single csv file (mac button &amp; i or rightclick and 'Get Info'). Open the 'Open with' apps list and you'll probably need to choose the 'Other' option at the bottom. Choose the 'All Application' selection and 'Always Open With' option while finding the Libreoffice app in the list and confirm that selection.

Now you'll have to click the 'Change all' button right under the App selection combo box on the general file info window. This step ensured all other csv files to open using Libreoffice from then on.