Visa exemption

Hi, i'm gonna go to Vitenam the 9th of janurary until the 16th janurary. At this date, i will pass the border of Cambodge, and after pass the border of Thaïland, all by train. So, my way back ticket is from Bangock. Is it problem that i don't have a return ticket from Vietnam ?
And also, do I need to apply for the visa ?