BookMyForex
Personally using

Buy USD as an extra emergency fund (usable for non-Vietnam trips as well)
BookMyForex is an easy-to-use online service for foreign currency delivery
Legal to hold up to $2,000 USD foreign currency per person upon return
Place the order at least 2 working days before departure
Ensure product type selected is Currency Note and NOT Travel Card
Order Proof & Delivery

Cost Breakdown: Order USD Cash vs VND Cash in India
| Metrics (Sample Conversion) | INR Amount | VND (INRVND) | USD (INRUSD) |
|---|---|---|---|
| BookMyForex Rate | ₹1,000 | ₫222,222 | $12 |
| Real Forex Rate | ₹1,000 | ₫287,845 | $12.15 |
| Conversion Cost | - | ₫65,623 | $0.15 |
| Conversion Cost % | - | 22.8% (Extreme Markup) | 1.2% (Low Markup) |