Why does java fail to stop a running openvpn process?

I'm using junit for a number of integration test that require different openvpn-connections. Within my tests, I start the openvpn-connection with the ProcessBuilder-class:

Process p = new ProcessBuilder("sudo", vpnBinary, clientfile).directory(tempDir.toFile()).inheritIO().start();
vpnProcess = p;

To close the connection at the end of the test, I add a hook during the test setup:

public static void setUpBeforeClass() throws Exception {
    Runtime.getRuntime().addShutdownHook(new Thread() {
            public void run() {
                stopVPNConnection();
            }
}

I tried different versions of the stopVPNConnection method, non of which works in a reliable way...

First:

private static void stopVPNConnection(){

        if(vpnProcess != null){

            LOG.info("Going to shutdown special vpn and start normal vpn");

            vpnProcess.destroy();
            LOG.fine("Destroy signal sent");

        }
}

Second:

private static void stopVPNConnection(){

        if(vpnProcess != null){

            LOG.info("Going to shutdown special vpn and start normal vpn");

            vpnProcess.destroyForcibly();
            LOG.fine("Destroy signal sent");

        }
}

The third made use of Java tool/method to force-kill a child process

The fourth starts another process:

Process p = new ProcessBuilder("sudo", "/usr/bin/killall", "sudo").inheritIO().start();

Up to this point, none of these have stopped the openvpn-connection in a sufficiently reliable way for automated testing.

Is there a better way to stop the process?

How to go about understanding why openvpn is not stopped?