It sounds like you want to build a parallel Sun Cluster while VCS is active. WOW! My initial reaction would be no. But.... Why not? Since you're going to be using on a completely different storage frame. You would be looking at a second set of resources, such as VIP's. You already have the SAN. Even if you're able to get fairly far into the build, you won't be able to test it very well. Honestly, I don't know the answer you're looking for. If it were me, I would start by disabling and shutting down VCS. If they can't give you a maintenance window to do the work, do they understand the risk of having this box sitting there like a dead fish? Be sure to break the mirroring so you have a quick way to get back to a running and healthy cluster.
"I" still would not go this route. You will most likely create a lot more headaches than what you may be prepared for.
Is it possible for you to take one of the nodes out of the VCS cluster and build that one as a Sun Cluster? Easily done if you have a active/passive cluster. Then you would have two one node clusters. Your VCS cluster can keep your production apps up and running during your build of the Sun Cluster node. Once finished, you can add the VCS node to your Sun Cluster node.