If there are no updates available, show a normal message box.
If there are updates available, open the VulnDetect website with the scan results (the list of applications) because there you show a message at the top, that the VulnDetect Agents needs an update and also a link to the new client (one-click-update).
Maybe you automatically log the user in (so the user does not have to enter the login credentials).

For business users this feature (checking for Updates of the VulnDetect Agent) should be configurable.
So the administrator can allow of forbid this feature.