I just tried the same thing and it seemed to work fine. My guess would be a permission problem on the files. Can you do "ls -lart /Library/Audio/Plug-Ins/Vamp" and post the results?
Last login: Wed Nov 22 16:42:48 on console
Vics-15-Retina-2:~ victordasilva$ ls -lart /Library/Audio/Plug-Ins/Vamp
total 3128
-rw-r--r--@ 1 victordasilva staff 48658 7 Sep 2015 qm-vamp-plugins.n3
-rw-r--r--@ 1 victordasilva staff 661 7 Sep 2015 qm-vamp-plugins.cat
-rwxr-xr-x@ 1 victordasilva staff 1547456 9 Sep 2015 qm-vamp-plugins.dylib
drwxr-xr-x 7 root wheel 224 22 Nov 16:40 ..
drwxr-xr-x 5 victordasilva wheel 160 22 Nov 16:41 .