https://en.wikipedia.org/wiki/Visa_policy_of_Vietnam* - A gap of at least 30 days between two visa-free visits is required. Must have a printout of onward ticket.
Note Vietnam's visa policies are in a state of flux. Always double check.
In less than three years 12 month visas for some foreign nationals have moved from around $300 to $900 or simply 'not available'.
Many of the Pattaya/SHV Refugees have found themselves doing quarterly visa runs from Vung Tau.
The onward ticket requirement is not always enforced.