You need to specify the desired size (85G) of the vmdk rather to the delta (10G).
Type man vmkfstools for details.
EDIT: Do not extend a disk that has a snapshot, you must commit all existing snapshots before modifying a VMDK
vmkfstools -X 10G disk.vmdk
You have to use the final total size, not the size you want to add.
VCP 3 & 4
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