You can take a bus from Hanoi to Vientiane, but bear in mind that it's a 24 hour ride - a night and a day. A few years ago, reports from travellers said that this was a rough ride on poor roads though mountainous areas, but offset by spectacular scenery so well worth doing. Latest reports suggest that things have improved, with better roads and a more modern bus with sleeper bunks.
*How to exit Vietnam and entry Laos?
There is a $1 "stamp fee" when leaving Vietnam and a $2 "stamp fee" when entering Laos. The bus assistant collected all the passports and money for these fees the night before and took them to the officials for those who already had visas. Foreigners without visas walked their own passports through the process..