South China

You can easily do it on your own, no need to to through an agency. It's hard to say how long it will take, since so much is depending on what you like to see and do, but no less that two weeks to see a few of the major sights between the border and Beijing, I would say. Two weeks will leave you time to stop at, say, Kunming, Guilin, Xi'an and Pingyao. Not that these places are necessarily for you, just an example.

It can take as long as you like, as long as you have, or as long as you don't run out of money.
Yes, you can DIY.
Cheers!

Timeframes and interests would help in making suggestions.

Regarding travel south, most people of this travel forum do things independently. Regarding how long such a trip will take, how long is a piece of string. It will depend upon your interests, what you want to see and how much time you have. Both Vietnam and Laos are easily reached from China though through different points. And of course, it is possible to exit China from one and enter through the other.

Regarding the TransSiberian, if you intend to go through Mongolia, then I do recommend going through a travel agency to get the ticket for the first leg from Beijing to UB because there are only 1-2 trains per week and they are generally booked with traders and travelers who use firms specializing in this route. Try contacting Richard Tian, whose email is oceantravelcn@yahoo.com.cn or richards@public3.bta.net.cn. Reportedly he is quite reliable about obtaining the tickets at a fair price and delivering them during the day to your hostel or hotel in Beijing. Be sure to add him to your address book so that your spam filter does not "Eat" his response.

If you have a Chinese residency permit, you can obtain your Russian and your Mongolian visas easily in Beijing. If you don't have a Chinese residency permit, I do not recommend Beijing for the Russian visa because people without permits have been refused visas at the embassy.

Generally speaking if people take the transMongolian train route, there are two patterns of travel. One is to go quickly without stops which takes about a week. The other is to stop along the way. See below for details but generally people spend 2 weeks in Mongolia so that they can get outside UB, perhaps to the Gobi or north to Khovsgul and then one month is Russia which is the maximum amount of time for which you can get a Russian tourist visa.

Try the library or a used bookstore for either of the two TransSiberian guidebooks, one by LP and the other by Trailblazer.

Look at these websites for starters:
waytorussia.net
Man in Seat 61
everbrite's travel pages
New Russian Train sticky
For train schedules, try these sites:
Die Bahn (multiple languages including English but no prices)
Your Train - CIS train schedule (Russian and English but no prices - if you don't know the spelling, just put in the first letter or two and there is a pull down menu)
Another Russian Train Site (only in Russia but tells you if seats are available within 30 days of travel and it gives the prices for available seats on travel within Russia.)

Usual stops in addition to Irkutsk to see Lake Baikal include any or all of the following (from Moscow to the Mongolian border):
Yekaterinburg which is worth a day to see the old center of the city and the new church built to commemorate the Romanovs. Also get someone to drive your to see the cemetaries where Yeltsin's parents are buried and take a look at the local mafioso graves.

Novosibirsk which is worth a few hours to see the Stalin architecture and if you are in need of an Irish pub but otherwise can be missed in my view.

Krasnoyarsk which is worth a day or two. Be sure to see the dam at Dvinogorsk and to go for a hike in Stolbi, the national park. The ethnography museum in the city is very good as well.

Irkutsk which is worth a day on its own and then several days in the Baikal area. Just outside of Irkutsk visit the wooden architecture museum to get a sampling of old buildings. Visit the lake front. If you have time, get some rest and solitude on Olkhon Island or visit the hot springs near Arshan in the Sayan Mountains, or take a ride along the circumbaikal train to see the lake up close.

Ulan Ude which is worth a day for the Buddhist datsan outside of town, the bust of Lenin and the ethnography museum.

Other stops that are also highly recommended but a bit off the usual path:
Kazan which is worth at least a day to see the Kremlin (A UNESCO world heritage site), the university that Lenin attended and walk along the pedestrian mall in the center of town.

Tobolsk which is worth a day to see its kremlin and some of the old houses.

Tomsk a university town which is worth a day to see the wooden buildings and soak up some atmosphere.

Ruth

i just did it on my own. it's not tough.

I don't know if this helps: Vietnam to China It's one way and at least tells you the border towns.