Round up taxi fares and leave 5–10% at nicer restaurants. Street food vendors don't expect tips at all.
Many upscale restaurants add a 5% service charge plus 10% VAT — check the bill before tipping again. Hotel porters appreciate 20,000–50,000 VND per bag.
For spa massages and tour guides, 50,000–100,000 VND per person is standard if you enjoyed the service.