How to run nginx.service after /vagrant is mounted

Using the mount.target unit seems to work.

Vagrantfile:

# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|
  config.vm.box = "ubuntu/wily64"
  config.vm.provision :shell, path: "bootstrap.sh"

  # Disabling mounting of the /vagrant directory to make sure ngnix is blocked
  # Comment this out to allow the shared folder
  config.vm.synced_folder ".", "/vagrant", disabled: true
end

bootstrap.sh:

#!/usr/bin/env bash

apt-get update
apt-get install -y nginx

# Make vagrant.mount loosely dependent on nginx.service
sed -i 's/WantedBy=multi-user.target/WantedBy=vagrant.mount/' /lib/systemd/system/nginx.service
systemctl daemon-reload

# Update service symlinks
systemctl disable nginx.service
systemctl enable nginx.service

Test this by running vagrant reload to cause the VM to boot without the /vagrant directory mounted. Log into the VM and see that nginx isn't running:

vagrant@vagrant-ubuntu-wily-64:~$ sudo systemctl status nginx.service 
● nginx.service - A high performance web server and a reverse proxy server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: inactive (dead)

Then comment out the Vagrantfile line that disables the shared folder and vagrant reload again. Log into the VM again and see that nginx is running:

vagrant@vagrant-ubuntu-wily-64:~$ systemctl status nginx.service 
● nginx.service - A high performance web server and a reverse proxy server
  Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
  Active: active (running) since Mon 2016-07-25 04:28:00 UTC; 42s ago
 Process: 1152 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
 Process: 1146 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
Main PID: 1156 (nginx)
  Memory: 7.4M
     CPU: 17ms
  CGroup: /system.slice/nginx.service
          ├─1156 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
      └─1157 nginx: worker process

edit /lib/systemd/system/nginx.service and change WantedBy directive under Install section...

WantedBy=vagrant.mount

You can check if the vagrant.mount event exists via: systemctl list-units vagrant.mount. You should see this:

UNIT          LOAD   ACTIVE SUB     DESCRIPTION
vagrant.mount loaded active mounted /vagrant

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

1 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.