Buses run from various destinations within Yunnan, including a wild overnight sleeper from Kunming (¥109). Some connections, including Jinghong require you to change buses in Mengzi. The bus station is within walking distance of the border post, and Lao Cai is just across the bridge. The border is open 8AM-11PM (7AM-10PM Vietnamese time).
When you leave the border office, ask the Chinese officials for a small guide with basic map of Hekou map and a glossary, if it is available.
Beware of helpful guys speaking English at the bus station, offer to sell bus/train tickets in Vietnam for triple their real price or exchange currency several times more expensive than in respectable places. Check prices and exchange rates in advance. Tickets can be bought easily at Vietnam side of border.