Usually only downhill trains are having technical halts at DDS but since a lot of people visit the falls trains like Goa, Poorna, Amaravati, etc take an unscheduled halt at DDS.
So I advice you to book a ticket from Vasco to Castle Rock and get down at DDS incase it doesn't halt (which will not happen normally) you can get down at Castle Rock and take a downhill freight train till DDS which would have a 100% halt there.