If i understand you correctly, the Volume is 1.8TB and the LUN on that volume is 1.8TB. (Typically you make the volume larger than LUN size - or LUN smaller than the volume in other words.) However, keep in mind, the LUN and any snapshot capacity will contribute to the used capacity on the Volume. Volumes are made up of luns and snapshot capacity in your case. On your volume you have 540GB free. If this capacity is overrun by snapshots the lun will go offline (depending on your configuration) So becareful - the best the to do would be to enable snapshot autodelete and volume autogrow. Now there seems to be a descrepency between the amount of free space on the LUN depending on which tool you use to look at it. Keep in mind that the netapp generally does not show an accurate size of free capacity because is works like a high water mark depending on versions of software on each side. I'd go by the host capacity so if ESX shows 644GB free - i would go by that. (*Note: Lookup space reclamation)
Now if you were to put a 300GB VM on that lun (lets just say it had less than 300GB free) you could still do that as long as you provisioned the Datastore or RDM lun as a thin provisioned lun. That way it would only take up the space it needed on day one instead of reserving all 300GB worth at once even if you didnt use it all up immediately. Thin provisioning is your friend as long as you monitor your capacity numbers closely.
I'm assuming your 1.8TB LUN is thin provisioned or else you wouldnt have any free space on the volume and you would be getting all kinds of alarms.