In your case, there are two options for you now:
1. Ignore your done 1 month single visa and apply for a new 3 month multiple visa prior to traveling to Vietnam.
2. Use your current 1 month single visa to enter Vietnam at the first time, then re-apply for another 1 month single entry visa online to return the country from Cambodia.
Hope this helps you. Should you have any other concerns about visa Vietnam, please do not hesitate to contact us at anytime.