Visa on arrival

Dear Sir or Madame,
I will travell trough south ost asia during april,may and march.
I don't know yet, when I will visit which country. I will start in Cambodia and than
persuite to vietnam (I won't take an airplain).
I saw that for a Visa you have to give a date of arrival and an ariport of arrival.
How do I have to persuit if I won't arrive in viatnam by airplane and don't know yet the exact date of arrival?

Thank you for your answer,
Kind regards
alessia