Hi everyone,
We are a family of 4, plan on traveling to Vietnam for a month in June.
Because we are quite a big family so can't design a specific plan to travel on our own. So we are looking for a professional, local understanding and private travel agency in Hanoi.
So if you have any suggestion for us would be really appreciated.
Thank you for your time.

Within a month, you can do it yourself easily in Vietnam (Slow travel and Less is more).
Do some research on Travelfish for which places appeal to you to make your own plan.

In addition to Tonkin Travel, Footprint, Exotissmo, Custom Vietnam Travel, Handspan, Ethnic Travel and Sinh Tourist are all worth considering if you really want to use an agency.

However, I agree with Din Ho that it can be done very easily and probably a lot cheaper yourself. All it takes is a little research, starting with an overall itinerary listing those places and activities that really interest you.

Once that has been nailed down (and you can get a lot of help with that here and on other forums). Once that is done you can then focus on transport and accomodation. Easily done online direct with the airlines and with booking sites like booking.com and Expedia etc.

One month is a pretty decent amount of time to explore the whole of the country but do pay heed to Din ho’s comment that "less is more". Being a long thin country, Vietnam rewards slow travel. The problem with travel agents, especially Vietnamese ones is that they always try to pack in too much and you are in danger of rushing around seeing very little. If you do use one. Have a good idea of what you want to see and insist on a slow pace (esp important with children)

I'd like to point out that you could make arrangements using a variety of options, depending on whether you want everything locked down in advance or if you'd prefer leaving some until after arrival, as you go. I made some reservations in advance online myself, booked some through an agency in advance and also booked day trips at local agencies after arrival.

Many hotels have in-house agencies & are convenient to use, especially if you want to get the lay of the land before you book. It's often good to wait so you can see what's on offer, more variety & often less expensive than if you book in advance.

You may want to consider Hanoi Voyages. I had got recommended by a friend and last year, my family did a Cambodia-Vietnam tour with them. We were absolutely satisfied with their customized itinerary and professional service.

"Could be rainy in June" it will be in the far north ( I wouldn’t consider any trekking at that time of year), , but the best tim of year for visiting the central region, Hue, Hoi An Dalat Etc. South will be hot but should be too wet - still plenty of sunshine.