You should be able to connect to the vfiler context like this:
PS C:\> Connect-NaController dunn -Vfiler vfiler1
OntapiMajorVersion : 1
OntapiMinorVersion : 12
Protocol : RPC
Vfiler : vfiler1
Name : dunn
Address : 10.61.167.60
Port : 0
Credentials :
ValidateIncoming : False
ValidateOutgoing : False
Trace : False
PS C:\> Add-NaQuota -Type user -Target vf1user -Volume testvol -Qtree "" -DiskLimit 50g
Name Value
---- -----
Threshold -
FileLimit -
DiskLimit 53687091200
PerformUserMapping False
SoftFileLimit -
QuotaError NetApp.Ontapi.Filer.Quota.QuotaError
SoftDiskLimit -
PS C:\> Get-NaQuotaEntry
Line : vf1user user@/vol/testvol 52428800K - - - -
Qtree :
QuotaError : NetApp.Ontapi.Filer.Quota.QuotaError
QuotaTarget : vf1user
QuotaType : user
Volume : testvol