I don't know if this changes in v9.5 or not, but in v9.1 SP5 you can find this setting by opening the System Manager on your Orion server. Look under File | Advanced Settings. There is a tab on the Advanced Settings window called Node Warning Interval. This lets you configure how many seconds should pass before Orion marks a node as down. As far as I know, this is a global setting and will apply to all nodes equally.
The only way I can think to do what you're asking to do is by setting up a couple of UP/Down advanced alerts, one for your wired nodes and another for your VPN nodes. In an advanced alert, you can specify a dely in the trigger actions which will keep the alert from firing until the trigger condition has existed for the specified period of time. Let me know if this makes sense or if you have any additional questions.
Hope this helps.