I think for the air tickets, that is a reasonably good price already; I doubt you'd be able to find anything much cheaper...those are the only two airlines flying between Kunming and Laos and the better of the two airlines with more services and being a fairly large, experienced airline is China Eastern, so I'd recommend choosing them, although if Lao Airlines is cheaper or you prefer their service, by all means go for them.
Anyway, I'd recommend a one way flight from Vientiane back to Kunming, catching the bus down first; not only can you pace yourself better because the flight back is only 1.5 hours, unlike the unpredictable bus journey, but you also avoid the possibility of being forced to purchase a ticket out of Laos if attempting to travel on a one way ticket. I don't think this should be much of an issue and you'd probably be allowed to fly anyway, but I'm mentioning this just in case.
If you travel round-trip via air, then you can travel via minivan between Vientiane and Luang Prabang, stopping off in Vang Vieng and possibly even Xieng Khuong, which is on rte 7 off towards the Vietnamese border (Thong Hai Hin or Plain of Jars) if you are interested, followed by returning to Vientiane for your flight back.
One way air tickets to Vientiane should be about 1000Yuan+tax or roughly 1300-1500 incl. tax, and probably a similar amount if purchasing your ticket in Vientiane.
I also heard that there is (or will soon be) a regular flight (twice weekly or so) from Jinghong to Luang Prabang with Lao Airlines. This may be another option for you, but I doubt it would be much cheaper than flying from Kunming since you would also need to factor in the cost of getting to/from Jinghong from Kunming.
There are many good boutique hotels in Luang Prabang, Vang Vieng and Vientiane. In Vientiane, I recommend the Chantapanya Hotel. In Vang Vieng, I hear the D'rose Hotel, which also has a sister hotel in Vientiane is good. For all cities, try www.agoda.com for hotel bookings - there you'll find both guesthouses and hotels, and although you can probably just show up without a reservation, I think it gives you some peace of mind if you book in advance.
In Vang Vieng, tubing and watching tv, getting plastered on beer, whiskey and other things seem to be the main activities, but you should consider going to explore nearby caves and an area next to the Mekong for swimming with the locals instead. I think it's much more fun and worthwhile than hanging around inside the town with all the drunks.