I understand that people fro the UK can now enter Vietnam by just turning up and getting a 2 week entry on arrival without the need to fill out any visa forms?

 

I was wondering, can you travel there on a one way ticket? or does Vietnamese immigration ask to see onward travel?

It's probably not the Vietnamese you need to be concerned about, it's highly likely that the carrier wouldn't let you fly without the required return ticket as they would be responsible for your repatriation if you were refused entry and maybe a fine. 

Vietnamese Immigration on arrival will ask to see proof of return or onward travel if there's no visa being presented. I lieu of having a printed copy, they will accept viewing the return/onward booking on a smartphone or tablet.