I have a tour in vietnam which starts in Ho Chi Minh city. I was planning on getting there as cheaply as possible from Kunming. Any advise? Dont know whether its a good idea to get to cambodia and travel overland to Ho Chi Minh City. But bearing in mind ill only probably have two weeks in between kunming and my tour i dont know if this is doable?

This trip certainly doable, albeit a little rushed. Now if memory serves me correct the railway line between Kunming and Vietnam is closed so see if you can catch a bus across the border. Would be worth stopping just over the border in Lao Cai and then catching a bus up to Sapa for 1 or 2 nights.
From this point you are on the tourist trail so buses and trains will run from Sapa - Hanoi - Hue - Hoi An - Nha Trang - Mui Ne - HCMC everyday. As a general rule buses are always cheaper than trains but for the overnight travel times the trains are more comfortable.
A couple of nights in each place and you've got yourself a nice trip between Kunming and HCMC. You should be able to get a visa at the vietnamese embassy in Kunming as well. Hope this helps

Kunming- Hekou on the sleeper bus. Cross border, get bus/train to Hanoi, take Jetstar domestic flight to Saigon, bookable on line. Shortest, fastest and cheapest way.
With 2 weeks easily doable.

ok am gonna leave kunming on 25th get to hekou 25th then get train same day to hanoi. then flight to saigon the next day. do you know if i can get a train ticket when i cross the border for the same day?
i know this is really close but my visa doesnt start until 25th and i need to be in saigon for 26th.

ok am gonna leave kunming on 25th get to hekou 25th then get train same day to hanoi.
How are you going to do that? I'd leave Kunming on 24th.

however, i was thinking it may be easier for me to leave yunnan ealier than expected and go to nanning to get the train from hanoi there. that way i can see some of the sights round there to.

You can definately catch a train to Hanoi on the same day after leaving Kunming, provided you leave early enough. It should take about 8 hours to the border, then you cross and catch a night train to Hanoi (leaves around 8.45pm, I'm not sure how many trains there are a day to Hanoi though or if there are later ones than that). You will arrive in Hanoi the next morning though, but it is certainly realistic to be on a train bound for Hanoi the same day as leaving Kunming, there is no reason you can't do so and not everyone has a year to travel in Asia, therefore it may be necessary to rush things sometimes. I know someone who has done the trip in reverse, he left Lao Cai at 8am China time after getting off the Hanoi-Lao Cai night train, arriving on the Hekou-Kunming bus at 8.40am, which departed at 9am and he arrived in Kuming around 4.40pm the same day.