Hi,  On your XtremIO box you will just need the journal volumes if you want them to come from that array. The journal volumes can come from either of the arrays at this site since you will have multiple arrays. In terms of size for failover, that technote doc will also talk about the changes in replication mode upon failback when you use snap based replication at the source and vnx splitter at the target so take a look at that so you know what mode to flip to in a failover. When it is snap based replication the XtremIO as a target still needs a journal, but it can be minimal for the config and be 10GB.  If this is an existing RP cluster there isn't a requirement for a repository volume because one already exists for the cluster, likely on your VNX. If you plan to remove the VNX from the environment then the XtremIO will need to hold the repo volume and you will have to migrate it there.  Let me know if this helps.
Hi All,  I have a few questions about integrating an XtremIO into an existing RPA / VNX environment. There is currently a single RPA system consisting of two four-node clusters - production and DR. The RPAs are replicating from VNX5600 (production) to VNX5400 (DR). A new XtremIO is being introduced in production and has to be replicated to DR VNX5400 using the existing RPA system. RPAs are running code 4.1SP2. As far as I understand, the following has to happen for the replication from XtremIO to VNX5400 to happen:  Appreciate your input. 
How to integrate XtremIO into an exisitng RecoverPoint and VNX environment