Currently there is no tool to measure the degree of fragmentation that exists in vSphere. And the only
utility to defragment a VMDK file is VMotion – to move the VMhome to another datastore and then SVM it back to the original datastore.
Above quote has been taken from Page 10 of following Technical white paper published by vmware
http://www.vmware.com/files/pdf/VMware-DynamicStorageProv-WP-EN.pdf
answers both your questions.