I finally figured out the issue myself. I was having two subscription with my account. And I was aware that to run the Cloud Shell in Azure Portal, we need a related storage account, I was having such storage account in my other subscription.
So my assumption is that Azure uses this storage account for all of my subscription to avoid multiple storage accounts for multiple subscription, for the same kind of metadata.
The issue was that Azure Portal is not smart enough to find which subscription I am using currently, at least it may be a bug. So all I had to do is to set the context to my subscription where I have the KeyVault.
Set-AzureRmContext -Subscription "subscriptionid"

Then I ran the preceding commands.
$keyVaultName = "az203vmencryptdisk"
$rgName = "az203"
$vmName = "az203"
$keyVault = Get-AzKeyVault -VaultName $keyVaultName -ResourceGroupName $rgName
$diskEncryptionKeyVaultUrl = $keyVault.VaultUri
I was able to get the output I was looking for.

Hope it helps.