Hi
If the connection servers are down then VDI is down.
If the vCenter is down then i dont think new provisionings will occur, so in my setup i have enabled deletion of VM when user logs off, and a new VM is provisioned, this may fault as i think its managed by the vCenter... Also, you'll not be able to create a new pools.
Couldn't tell you about composer as we are using Instant Clones
Hope that helps