Any opinions on whether the agencies that advertise on the web - such as Exottissimo - are better/worse/same as local agencies such as Sinh Cafe for day trips to Cu Chi tunnels and other similar tours?

For that matter, I welcome opinions as to 1) what day tours to take in Hanoi, Hue, Hoian and Saigon and 2) whether I should plan in advance for these? ( we're typically pretty whatever-strikes-our-fancy travelers, but if advance planning makes sense, we can be convinced)

Thanks.

Hard to give an opinion on which day tours. It depends on where else you're going. E.g., from Hanoi, we're not going to Sapa and therefore want to see Duong Lam and KySon village. But we are going to Halong Bay and therefore are passing up Ninh Binh/Tam Coc (both karst).
sf7307, I forget whether you are DIY or using a travel agency. If you have an agency you trust, ask for their recommendations. Or else put out an inquiry with your dates, etc. to Exotissimo, V'Explore, and Tonkin and see what they say--and what you like among what they offer.
We're in the midst of this process now, and advance planning helps. We were offered Cu Chi Tunnels, but wanted to see Cao Dai temple more. The agency came back with a tour that combined both.
From Hoi An I wanted not just a city tour but a trip to My Son Cham ruins. One agency offered that as part of their day tour around the city, the other didn't.
If you don't want to do the research and plan in advance, then you may end taking what you can get on the spot, deciding without due diligence--and that's a bit of crap shoot.

WillJame-Which company are you using for Cao Dai temple and Cu Chi tunnel. I need to book a day tour for March.

Shelley, this came from Nhung of Tonkin Travel: <<Cu Chi tunnel is on 2/3 way from Saigon city to Cao Dai temple. Cao Dai temple tour is from 8.30am till 3PM. If we do visit Cao Dai temple, we will be rushed for afternoon city tour in Saigon. To my idea, I suggest 3rd March to do 1 day tour Cao Dai temple & Cu Chi tunnel. We will move Saigon city tour to morning of 5th March before flight to Nha Trang. How do you think?>>
The earlier itinerary had Cu Chi Tunnel in the morning and a city tour of HCMC in the afternoon. This was Nhung's suggestion when I asked for Cao Dai instead of Cu Chi.

You can arrange day tours on the spot if you want them. The only day tour we took from Hanoi was to the Perfume Pagoda, and we just arranged a car and driver. In Hanoi, you can really do it all yourself - no need for a tour.

no need for tour in hoi an..

look at my trip report for the guides we used in saigon and hoi an....

No need for advance planning. We did a one day tour of Cao Dail Temple & Cu Chi tunnel through Sinh Cafe and it was fine, although it was a very long day. The time went by though as there were nice travelers to compare travel notes with during the long bus ride. We used Sinh Cafe also for a trip into the Delta. If you are short on time in Hue, do not take the boat to the Perfume Pagoda, but rather hire a driver through your hotel. They can take you at the same time to visit various Emperor's tombs. Hoi An does not require a guide. If you are interested in a guide, you can check out the Hanoi Kids or Danang Kids. I used the Hanoi Kids and was very pleased. I believe Bob used the Danang Kids.

Just returned from Cambodia and Vietnam with Vexplore doing our tours. Very pleased. In Hoi An really loved the Red Bridge Cooking School experience. Good value and delicious food in charming atmosphere.

Definitely no need to book ahead. Vietnam is overfull of agencies. You really don't need a guide for Hoi An (or, I would say, for Hanoi or Saigon either, but that's more a matter of personal choice). Just hire a car and driver for My Son - and go early to beat the tour groups.

So we're here and find ourselves with enough time to visit the Cu Chi tunnels. Agencies want anywhere from $10 to $45 per person for the trip (one said we could have a private driver/car for $38 -- theirs was $23 in a van). Is there anything significantly better about going the more expensive route?

By the way, we were wandering around today, and ended up in the courtyard of what turned out to be a military museum, near the zoo. Noone stopped us, but neither did we have any idea where we were -- it's not on any of the maps we had, nor is it discussed in the Lonely Planet guidebook. Anyone have any idea? It's two buildings outside the entrance to the zoo.

For Cu Chi tunnels you just need to get there. The guides are on-site, no need to bring your own. So I would go with the cheapest option.

If you're going to the Cu Chi tunnels, try to see Cao Dai temple as well - I preferred it.