Bus from Na Meo to Thanh Hoa - Ha Noi leaves between 12.00 - 1 pm. It's terrible bus but you'll be paying high price as tourist: I heard a French man was charged something like 700 000 VND for the ride to Ha Noi. We managed to get 300 000 VND per person for a ride to TH, still a lot but couldn't get for less. Vietnamese people pay less, also Lao. Once you're there you don't have much choice than taking that bus (or - some say you can go by motortaxi for even more money and even less comfortable way).
Fun Alternatives: Go by your own car or bike.
Opening time on the border is from 7.30 am - 4.30 pm, lunch break is between 11.30 - 1.30 pm.
If you're from one of these countries (Scandinavian passport) who can get Vietnamese visa on arrival make sure that you have airplane tickets with you which prove that you'll depart Viet Nam in 14 days. If not the Vietnamese border official will reject your entry and you'll have to get either air ticket or Vietnamese visa from embassy. I saw few people being turned down there and had to go back to Sam Neua - because there's nobody who can help you at the Lao side in the village and bribing the officials didn't help much either.
Another thing worth to be aware of: if the Vietnamese officials offer to change your KIP in VND you should better know the exchange rate in advance because they love to offer much less than is worth. However, change KIP in Nam Meo - further from the border you get, lesser the chances to change KIP in VND and worse the rate. Alos be prepared for careful check of your belongings by the same officials.
If you want to enter Laos from Vietnamese side you need to have Lao visa in advance (as for 2009), VOA for Lao is not yet available - hope that will change soon.
Another note on Lao side: you're required to do some sort of medical check - and the only thing nurse will really do is to measure your temperature. Pay 2000 KIP for that. And some more for the offcial, no matter what's the time if you wanna go smoothly through the passport control.