When you are in Sihanoukville and you’ve plans to go to Vietnam you can easily arrange your visa here. (better than do it in Phenom Pehn, because in Phenom pehn the visa will be send to Sihanoukville) You can arrange your visa yourself or let a travel agency do it. You can choose your own start date and how long your visa will be. One or three months.
Note: You can’t fix a visa on arrival so do it before you go to Vietnam.