Visa - simple or for more entries?

Dear Sir or Madam,

I need an information about visas.

I want to make e-visa for 1 month (tourist) for Vietnam.

I see that there are two categories of them: for one entry or for more entries.

I start my tour in Hanoi and will end it in Ho Chi Minh City. Maybe I will go also to Cambodia (Phnom Penh and Siem Riep) and will come back to Ho Chi Minh City, but I don't know now if I will have enough time to do Cambodia.

My problem is: whether to take visa for simple entry or to take it for more entries. I think I will not have time to go to Cambodia, but I'm not sure now.

If I take now visa for simple entry, is it still possible to go to Cambodia and come back to Vietnam? What kind of formalities will I need to do in this case and for what kind of prices if this is possible?

Thank you for your answer!

Kind regards,
Tjaša Šeneker (from Slovenija)