Is there a minimum stay outside Vietnam before reentry?

Hello,
My husband and I are planning our Vietnam holidays right now and would like to go to Cambodia for a few days to see Angkor Watt before returning to Vietnam for the last week of our holiday. As we will be traveling a total of 3 weeks, we would apply for a multiple entry visa.
Is there a minimum stay outside Vietnam (Cambodia) before we are allowed to renter Vietnam?
We'd be travelling by plane.
Thank you for your help