You can access the .vmdk file by browsing the datastore options in the VI Client but this requires root permissions.
It would not be possible to fetch the .vmdk file through the api's but some of the content can be fetched using the api's. You may browse to the VirtualDisk dataobject, VMFSDataStoreInfo and VMFSDataStoreOption. Please refer the VI SDK Refernce Guide for specific details.