Is it cheaper to book separate one-way tickets?

Not usually. Multi-city tickets often have better 'fare construction' rules that avoid the high cost of one-way international tickets.

What if my budget airline flight is delayed?

If you booked them as separate tickets, you are on your own. Always leave at least 6 hours of 'buffer' time between separate flight bookings in Southeast Asia.

Booking Multi-City Flights for Southeast Asia

Use the 'Multi-city' search tool on a flight aggregator instead of booking separate one-way tickets to save on baggage fees and ensure airline cooperation for delays. Aim to book your 'open-jaw' itinerary—flying into one city and out of another—at least 6 weeks before departure.

Map your primary hubs. Identify your 'entry' city (usually Singapore, Bangkok, or Kuala Lumpur) and 'exit' city. These hubs offer the most international connections and lowest long-haul fares.
Search using the 'Multi-City' tool. Open Google Flights, click the 'Round trip' dropdown, and select 'Multi-city.' Add your long-haul legs here to see the total price as a single booking rather than two separate tickets.
Check regional carrier segments separately. Once your long-haul flights are set, look for regional legs (e.g., Bangkok to Hanoi) on budget airlines like AirAsia, VietJet, or Scoot. Compare the cost of adding these as a multi-city segment versus booking them individually, as budget airlines sometimes don't sync with major aggregators.
Prioritize single-booking itineraries. If you book the entire route on one ticket (if possible through an airline alliance), the airline is responsible for rebooking you if a connection is missed. If you book separate tickets, you are responsible for any missed connections.

Is it cheaper to book separate one-way tickets?: Not usually. Multi-city tickets often have better 'fare construction' rules that avoid the high cost of one-way international tickets.
What if my budget airline flight is delayed?: If you booked them as separate tickets, you are on your own. Always leave at least 6 hours of 'buffer' time between separate flight bookings in Southeast Asia.