I not only agree with Jeff, but let's take it to another level. Sometimes you can use a phillips or a flat-head interchangeably, which is the case sometimes with Virtual vs Meta. But many a time you can't interchange them. I've been learning a bit about virtual directories lately thanks to Mike Brengs and his team from Optimal IDM. Here are a few differences to share right now.
When to use a virtual directory:
1- Use it as a proxy. What if you aren't allowed (or can't) query directly against AD. Meta directories shouldn't do this.
2- Extend the schema. Yea like ADAM, but it won't take weeks to set up.
3- Creating single views that update immediately. Similar to meta directories, data can be brought together in many different way. Unlike meta directories that poll connected sources in intervals, virtual directories can meet client requirements that need immediate results.
Here's something that meta directories must be used for and not virtual directories: provisioning and de-provisiong user accounts. This is one of the main pushes for meta directory technology and it still holds true, especially de-provisioning. Now some vendors have added a sync engine to their virtual directory product so it can do both. Which is fine, but then call it what it is.
As I continue to understand more about virtual directory technology I'll include it here.
Does anyone agree / disagree or can add more to this?