I use DNS discovery for our APs deployed on customer sites. I use an A record pointing zonedirector.localdomain to the external IP of the vSZ. As long as you have the ports open on vSZ site the AP will end up in staging zone. I recommend reading the administrator manual for the vSZ. It describes all this. There are also documents on the support portal where you can find all the necesary ports that you will need to open so your external AP:s can communicate with your vSZ.