For all travellers, price of air ticket comes to mind firstly. To book a reasonable flight tickets, here are some tips:
If you want to buy a ticket on website, here are some of popular ones, namely: Skyscanner, Expedia, Kayak or TheFlightdeal, etc. We recommend to check and compare offers to get the best deal. As a research, the best day to catch a good deal is on Tuesday, the price of ticket is normally going up and expensive on Friday and on the weekend. Remember to clear cookies for each searching. 
If you have a pretty good budget and do not want to spend too much time moving among airports, you can search for direct flight to main airports like: Ho Chi Minh airport and Noi Bai airport. You will have more time for travel and discover the destination than sitting at lounge in the airport.
If you are seeking for a lower price ticket, be prepare to book a connecting flight with stops in one of the international airports in Asia such as Hong Kong, Singapore, Bangkok, Doha,etc before get a flight to Vietnam. Another options, you can also ask and check price at travel agencies near you.
Luggage allowance:
Each airline agency has different policies for luggage allowance.
For economy ticket, the carry- on bag allowance is normally is 7kg, and the check- in bag is 20kg, however, for some low cost airline, the price of check- in bag will charged and invoiced separately. Pay close attention to suitcase weight because it will be charged athigh rate for excessive weight at the airport.