I feel I know VN quite well, but haven't been on a tour myself since 1993. So really what I know just comes from hanging around a few touristy places and asking how people have got on or arranging stuff for visiting customers etc.

Your comment re. guides is interesting. I can well imagine that the Saigontourist guides are more reliable. The private companies are variable - the reality is that most of the guides are freelance and just move between companies as required.

In 1993, I used a UK company who hooked up with "Vidotour" in VN. I think it was meant to be the first privately run tour company in VN. It was also run by a very professional/energetic woman. Everything was perfect - very knowledgeable guides who spoke excellent English (common now, but not common in 1993 in Hanoi). It was a 3 week tour from North to South. Vidotour are still going and seem to be a big player now. I'd certainly consider using them http://www.vidotourtravel.com/ (Noticed they've got a Scottish bloke as a Chief Financial Officer - no doubt he can trace a T/T!)

Ann Tours are often recommended here. Another I've mentioned is http://www.threeland.com/. A friend had a less-than-perfect guide (a Chinese speaker, at her request) who was lazy. They immediately replaced him on getting a complaint. I think this is as much as you can hope for...

Once in country, Sinh Cafe are well known for helping backpackers - but can book posher hotels/private cars etc as required. Also Fiditourist (Nguyen Hue in Saigon). They seem to be less used by foreigners, but I've found them reliable and professional for organising trips to Mui Ne etc, hotel bookings etc.