There are no commandline options for ESXi to convert / create a vmdk from an Iso-file.
For Linux there exists a tools named isohybrid.
It was designed to eaily create an USB-bootable stick when you have a bootable ISO.
This tool can be "abused" for your case:
isohybrid path-to/iso
When the tool has finsihed the iso is slightly modified so that it boots after the image has been transfered to a harddisk.
In the ESXi environment an iso-file that has been modified by isohybrid can be directly used as a bootable vmdk.
The iso-file still has to be renamed to name-flat.vmdk and you have to create the descriptor.vmdk manually.
Though I use this quite often it is not a common scenario - never heard about anybody else using iso-files like this.
So to make it short: normally there is no need to convert isos to vmdks - because it does not make sense unless you have very special needs.
The vmdk created with the workaround I mentioned is not compatible with any standard partitioning tools - you can not write to it easily and lots of additional problems turn up.
The only useful usecase for an ISO - to - VMDK converter tool is if you have a Linux-LiveCD and want to boot this one file in as many different scenarios as possible.
I am sure that most VMware admins would tell you: iso to vmdk is as useful as apple to oranges.
It is neither useful nor does it make any sense.
Please explain your special usecase.