The “make_output.txt” file, present in one of the previous posts above, suggests that “libROOTVecOps” was not present in “ROOT_LIBRARIES” (which originates from “ROOTConfig.cmake”, I guess).
For whatever reason, all ROOT libraries were hardcoded with “-Wl,-rpath,/.../lib” and then adding “-lROOTVecOps” did not work (maybe adding ROOT::ROOTVecOps to target_link_libraries would do the job, though).