A question about visa

You don't need 2 ways ticket despite it saying somewhere that you need it.  I've traveled to Vietnam in the past 15 years at least 10 times. Never once have I been asked to show my travel itinerary.

That being said you can go to a Vietnamese travel agent and ask if they print out 1 way ticket itinerary showing as 2 ways. If anything Vietnamese travel agents will get you cheaper tickets to Vietnam than any online site.

I've flown into Vietnam before with a one way ticket using a VISA, 2 week VISA free and Visa exemption certificate and have never been asked to show an outbound flight.  Some countries are strict with this and I have been asked at immigration before, just never in Vietnam.

You don't even need to show an itinerary to get the Visa in Vietnam. Like my GF flying to UK needs to show one to get a Visa. It's easy though, just go to any airline website directly that gives you the option to pay in cash instead of online. Emirates is good for this, if you select pay in cash to their account, they hold the ticket and email you a booking confirmation and itinerary with the passenger name on and instructions on where and how to pay. Print this off for your application and the booking automatically gets cancelled in days. Once you get the visa, book any one way or return flight you want.

The return ticket 'urban myth' may well have applied
somewhere, sometime in the dark past, but not now.

Yet strangely, you can (often) find a budget airline
offering return tickets for less than 1/2 price, so do
your research well.  The above answers are all valid.

Best wishes.   And welcome to Heaven on Earth.

As Bazza said, sometimes there are factors other than price.  When I went to Vietnam with the intention of staying a while, I could have bought a one way on another airline but ended up with a round trip (return for you English speakers) ticket on Philippine Airlines.  My deciding factor was that their excess baggage policy was a lot better and at that time they still were going with the 70 lb limit.   I had six full weight bags so it added up to enough to make it a better deal. 

I can't cite anything but I think part of the airlines reluctance to provide one way tickets is that international treaties may demand that they return you for free if you get thrown out for overstaying your visa and have no money for a ticket.  It also may be determined by factors such as what passport you hold or what country the ticket originates from.  We Americans are notorious visa jumpers. 

Elena, I give you the best trick which works every time. When asked for a return ticked by immigration officer, you say you come to Vietnam to seek medical surgery. And you have no idea how long the diagnostic result and doctors' orders are so you can't decide before hand.

You won't even be asked in Vietnam.

Come to think of it, the only times i've ever been asked by anyone is when I have been in transit with a connecting flight. If it's a long time or sometimes overnight and I need to leave the airport. In Sri Lanka and even one time in Singapore, they've asked to see a connecting flight. This makes sense, because there are transit visa's in some countries and I guess they have to make sure you're going to be able to leave the next day.