You could have an issue with the airline who flies you in. They will all to see the ticket. If it doesn't match the visa time, in worst case scenario, they can raise to fly you. The airlines always want to see your exit ticket.
I buy a low cost flexible local ticket, usually Phuket to Singapore for example. If we decide to stay longer we just change the ticket. If we don't and go out within 30 days we get a refund or use the ticket. You need to show an out ticket to the airline that brings you in.
Hanoi was very easy for us. Just walked in with all the requested info including 6 months of bank statements, not 3 months, full flight out with original invoice and an itinerary of what we wanted to see in Thailand during the stay etc. 2 days later we picked up our visa. They were very nice and helpful. They wanted payment in $s for some reason.
bought through a very well-respected international 3rd party. I actually wrote to the CEO of that company and every board member. 1 year later they dropped AXA as the underwriter of their policies.