I wasn't going to answer your post as I don't know the answer, but seeing as no-one else has I will just chip in and say that I think it's unlikely that a bus runs through the border. It's more likely that you will need to get to the border in Lao Cai and pick up a bus on the other side. It's rare for buses to cross borders. Also, I've never noticed Yuanyang on the departures board at Lao Cai bus station but if there is one it's likely to run from elsewhere in town, the train station maybe. Sorry I can't be more help.