The above image was created by Daniel Feller, a Lead Architect for Citrix.  The blog post can be found:

http://virtualfeller.com/2013/10/29/just-the-apps/

Before you decide to rush forward with a Hosted VDI solution.  It's worth reading over this posting by DJ Feller.  Xendesktop 7.1 App Edition (Or Xenapp) can host 500 users "with just three physical servers using two Intel Xeon E5-2690 @2.9GHz with 192 GB of RAM".  You'll notice that to pack the same density of users into hosted VDI, you're looking at perhaps 500VMs, in comparison with 21 VMs when using the hosted shared desktops/apps method

I can see the point in VDI, but in many cases a hosted shared desktop solution will work (Xenapp). So  establish whether your user base needs a full fat hosted VDI solution. Do users care whether they see their Desktop? or do they just want reliable access to the tools that allow them to perform their job?