I know where the function is to do this ...
In order to be able to find out how much time it takes, please provide the information what exactly you are thinking of. Although Eager Zeroed may have a performance benefit, this is probably not noticeable with the VNX storage system in place. I'd actually go ahead an inflate the virtual disk from e.g. the Datastore Browser, which shouldn't take more than a few seconds.
... was restored as thin provisioning thinking it would take less time to restore.
The format shouldn't make a difference. It's the amount of data, and how it is restored which takes time.
André