I have had a lot of questions over the years around running replacing Citrix XenApp with VMware View, and I love this talk track, but unfortunately this far too often become a religious battle as the existing Citrix team struggles to retain their relevance by trying to illustrate how virtual desktops don't meet the efficiencies of server based computing... Interestingly, these are the same folks that are later far too willing to try out XenDesktop, but that's another discussion.
I would far prefer to undertake the discussion that they ought to be virtualizing the existing Citrix farm to get the efficiencies that come along with VMware. The Citrix team will still push back, and they will try to P2V a couple systems, inevitably they run into problems and declare it untenable.
So, first of all, why would you virtualize a XenApp farm?
In short, there is a lot more to the story than just consolidation. I have customers that have deployed VMware ESX server for their high-end Oracle databases, and they have accepted a 1:1 consolidation ratio (that means one virtual machine per physical host). The value to the organization is not just in consolidation, but in the strength of the platform, the disaster recovery capabilities, and how the infrastructure immediately becomes more fluid and resilient.
The next question is how would you virtualize a XenApp farm? To properly size a virtualized Citrix farm, you must start small, perform your baselining, and scale out untill you figure out where it breaks. Most customers miss this point, assuming they can just P2V their existing farm and get the same or better performance... These same customers, however, tackle Tier 1 applications appropriately (Exchange and SQL), and take the time to size their virtualized solution appropriately. The same must be done for XenApp.
For your reference: