I've got to be honest, I'm not a big fan of this design. Whenever I'm virtualizing resources for production environments, I prefer to have two virtual I/O servers. But, as you say, budget was a consideration. Still, I can't help but to think for about the same cost, you could have gone with the quad core POWER 5+ in a p561 with two system drawers, and have been able to host all of your partitions in one server with the redundancy of two VIO servers and more hardware level redundancy.
But no use crying over spilled milk
With what you have, I really don't see a better way of doing what you propose. The memory requirements of your Production ERP server limits your flexibility.