Should I use a travel agent to book these flights?

Not necessary. Use Google Flights to research and book directly through the airline's website. If there is a flight cancellation, it is significantly easier to deal with the airline directly than through a third-party OTA (Online Travel Agency).

How much time should I leave between connections?

If you are on two separate tickets, allow at least 6 hours. If you are on one itinerary (same airline or codeshare), 2-3 hours is usually sufficient at major hubs like Singapore (SIN) or Bangkok (BKK).

How to Book Flights for a Southeast Asia Couples Trip

Book a multi-city ticket to fly into your first city and out of your last city to avoid wasting time and money backtracking. Use Google Flights to track prices 3 to 5 months out and prioritize airlines that allow free changes or cancellations.

Use the Multi-City Search Tool. Never book two separate one-way tickets on different airlines if you can avoid it. Use the 'Multi-city' tab on flight aggregators to build your route (e.g., Fly into Bangkok, fly out of Hanoi). This is almost always cheaper than separate tickets.
Track Prices with Alerts. Once your dates are set, set up a Google Flights alert for your specific route. Monitor these for 2 weeks. If the price drops by more than $100 per person, book immediately.
Avoid the 'Basic Economy' Trap. Southeast Asia flights are long-haul. Ensure your booking includes at least one checked bag and seat selection. Many ultra-low-cost carriers in Asia charge aggressively for these extras; it's often cheaper to book a standard economy fare upfront.
Check Regional Low-Cost Carriers. For hopping between Southeast Asian countries, look at AirAsia, VietJet, or Bamboo Airways. Book these separately from your long-haul international flights, but leave at least a 6-hour buffer between arrival in your hub and your next departure in case of delays.

Should I use a travel agent to book these flights?: Not necessary. Use Google Flights to research and book directly through the airline's website. If there is a flight cancellation, it is significantly easier to deal with the airline directly than through a third-party OTA (Online Travel Agency).
How much time should I leave between connections?: If you are on two separate tickets, allow at least 6 hours. If you are on one itinerary (same airline or codeshare), 2-3 hours is usually sufficient at major hubs like Singapore (SIN) or Bangkok (BKK).