I have migrated a cluster a while ago, this went without any problems.
The load is divided per VS, so when you have 2 heavily loaded VS's with VPN's you could make sure those are split over the 2 cluster members.
Depending on the version you can see a lot of things with cpview, here you can see the statistics and the load per VS.
On the management server you can set the weight of each VS using:
vsx_util vsls
You can also assign specific VS's to a specific cluster member.
See the VSX admin guide for the version you have.
Regards, Maarten