Hi Rahul,
This should should be fairly simple.
What you need is - script on the vRA to monitor the disk usage
vROPs epops agent install on your vRA - Install the Agent on a Linux Platform from an Archive
Go to the vRROPs, find the vRA as Linux VM, add the script and configure the notifications as alerts, triggers, custom groups, emails and so on.
Cheers,
Petko Minkov