After having copied the plugin on the Vault's plugin directory and having registered it (everything worked so far), when you enable the secret engine an error occurs:
$ vault secrets enable enigma
Error enabling: Error making API request.
URL: POST http://localhost:8200/v1/sys/mounts/enigma
Code: 400. Errors:
* invalid backend version: 2 errors occurred:
* fork/exec /usr/local/libexec/vault/enigma.1.0.0: no such file or directory
* fork/exec /usr/local/libexec/vault/enigma.1.0.0: no such file or directory
Please don't get confused by the "no such file or directory" error, the plugin is there named correctly enigma.1.0.0
$ kubectl exec -ti vault-0 -n vault -- ls -l /usr/local/libexec/vault/enigma.1.0.0
-rwxr-xr-x 1 vault vault 15803651 Feb 21 08:51 /usr/local/libexec/vault/enigma.1.0.0
I supposed this is related to the fact that Alpine Linux relies on MUSL LIBC instead of the GNU LIBC.