The issue isn't vCenter per se, but rather the VSA Cluster Service. By default this is installed on the vCenter server. This provides the third vote in a 2 node cluster.
Therefore, if your vCenter & Cluster Service are running in a VM on one of the ESXi hosts participating in the VSA cluster, and that host fails, you lose two out of your three votes. Because you do not have a quorum of votes, the cluster will go down entirely.
You should place your vCenter server & Cluster Service on a seperate host.
Alternatively you can decouple the Cluster Service from vCenter (5.1 only), have vCenter run on the VSA cluster, but have the cluster service running outside of the VSA cluster. To be honest, it is easier to have vCenter & the Cluster service together in the same VM outside of the VSA Cluster.
There are a number of blog posts on the vSphere blog which discuss your options - would be worth having a read of them - http://blogs.vmware.com/vsphere/2011/10/vsphere-storage-appliance-vsa-useful-links.html
HTH
Cormac