You can put the estimated arrival date, as long as it is the earliest date you plan to arrive in Vietnam. For instance, if you apply for one month visa and put the arrival date as March 01st, your visa would be valid from March 01st to March 30th. Therefore, you can enter Vietnam at any time within the period.
Should you have any further enquiries, please do not hesitate to contact us at firstname.lastname@example.org or call us at (+84) 909 343 525
Hannah Yan (Ms.)