Most of these contacts just happen informally. We just talk to our friends.
After I came back from Afghanistan, some months later I began getting hounded by emails to attend a follow-up appointment with a military social worker. After long enough, they were eventually able to pin me in a room and get me talking, just to make sure things were good, and they were. When you're still serving in the military, there is at least a mechanism to compel you to do that. Once someone is out, they can't be compelled. That said, once someone is out, there is no reason, to my mind, that someone couldn't just check up and say, “Hey, you've been out for a while. Has anything emerged since your release that you think might benefit from access to supportive resources?”
When you release from the military, you do not automatically become a VAC client. There are probably privacy firewalls somewhere in policies or regulations that preclude names and contact info from getting sent from DND to VAC. I can't provide a solution to that.
Hypothetically, that wall could be knocked down and VAC could be enabled, on a proactive basis, to reach out one or two years post-release just to say, “Hey, we're following up with you. You've been out for a while. How are you adapting? Do you know we have this and that?” Frequently, I find that veterans are completely unaware of the options that are open to them.
As to how this would be done, I would need to do a lot more thinking on that.