Set up a service to trigger FIRST (this would be the shutdown service):

# cat /lib/systemd/system/fakehalt.service

[Unit]
Description=Fake-Halt Service
After=fakevm.service
Requires=fakevm.service

[Service]
Type=simple
ExecStart=/usr/local/bin/fakehalt.sh #this will fail until fakevm succeeds
ExecReload=/usr/local/bin/fakehalt.sh

And then set up the one that you want to run and complete BEFORE shutdown is permitted:

# cat /lib/systemd/system/fakevm.service
[Unit]
Description=Fake Service
Before=fakehalt.service

[Service]
Type=simple
ExecStart=/usr/local/bin/fake.sh

Create a script to represent the VM shutdown (or any process that you cannot anticipate the duration of)

# cat /usr/local/bin/fake.sh
#!/bin/sh

test="1"
sleep 21
if [ X"$test" = "X1" ]; then
    echo "vm has shut down" > /tmp/fake.test
    exit 0
else 
    exit 1
fi

And a script to pass for a shutdown signal:

# cat /usr/local/bin/fakehalt.sh
#!/bin/sh

sleep 3
cat /tmp/vmfake.test > /tmp/haltfake.test

Start the service you want to happen AFTER the first one:

# systemctl start fakehalt

What "should" happen is that fakehalt will fail to find a file called /tmp/fake.test to cat from, and so everything should go horribly wrong.

What actually happens is that systemd places fakehalt service on hold until it gets an exit 0 signal from the fake service. So if you wait 21 seconds and cat /tmp/fakehalt.test, you see that the cat from a file that did not exist when fakehalt was started - actually succeeded.