Hi found an old bit of code...might be rickety but it seems to work - assuming you're using PDMW not PDMWE
It assumes that you've already connected to the vault..as MyConnection
Dim MyDoc as PDMWDocument
Dim errorcode
Set MyDoc = MyConnection.GetSpecificDocument("put doc name here")
MyDoc.TakeOwnerShip
MyDoc.Properties.Add("Name of property goes here")
MyDoc.Properties("Name of property goes here") = "New property value"
errorcode = MyDoc.Properties.Update()
MyDoc.ReleaseOwnerShip
As I said, this is old and needs tidying, but it should hopefully work.
Regards