Hi,
As I know, travelers go will family often book tour before arrival because they want to make sure their plan will go right. Group travelling is difficult to change the plan than solo or couple travelling. 
However, booking in advance is easy now and it’s more convenient for you. Travel agency or hotel can support you lots of information before your arrival likes destinations, prices, transportation or other services in Vietnam. Some popular websites as Tripadvisor or Agoda is very reliable for booking and they have many promotion. 
But if you dont feel ok with booking in advance, book on place is ok. There are so many travel agencies have offices around Hanoi (mostly in the center area). 
I can’t tell you all agencies and tour operators will satisfy you but here’re some suggestion if you need reliable ones:
– Hanoi kids – free tour around Hanoi running by students
– Discovery Indochina – one of the best travel agency, friendly, professional and they also have 2 hotels in the center http://discoveryindochina.com/