Visit HK + Day-trip to Macau

Some of my family members are Singaporean while some are Vietnamese nationals. We will like to visit HK for a vacation, during which we will make a day trip to Macau. I understand that those family members of mine who are Vietnamese passport holders would require a HK visa as well as Macau entry permit. Question is: can they re-enter HK on the same day using the same HK entry permit that they have applied for, or must they apply for another HK entry permit upon arrival after returning from the Macau day-trip? Please advise.