Even if you have some kind of clever VM storage device such as a Tintri, ideally you should still defrag your VMs. If you’re still running VMs on a legacy SAN you should definitely keep them defragged.
I’m not going to go into all the details here, because there’s a great article that explains it all over at Virtual Strategy. In short though, defragged files and consolidated free space require far fewer IOs and thus allow the storage subsystem to operate far more efficiently.
Personally I have a site licence for Raxco PefectDisk, and it does a great job.