Option 2: Any Bangkok-Nong Khai train + bus or taxi to Vientiane...
The other way to travel from Bangkok to Vientiane (or vice versa) is to take any train from Bangkok to Nong Khai, overnight trains 69 northbound or 70 southbound being the best choice. Nong Khai is just a few kilometres from the Friendship Bridge over the Mekong River into Laos. You then use road transport across the Friendship Bridge into Laos like this: (1) Take a local tuk-tuk taxi from Nong Khai railway station to the Nong Khai bus station. (2) A shuttle bus runs from the bus station across the Friendship Bridge to Laos every 20 minutes throughout the day, fare about 30 baht. It stops at Thai immigration 5 mins after leaving the bus station then crosses the Friendship Bridge to arrive at Lao customs & immigration some 10 mins later (a 30-day visa on arrival available for Laos at this border point). (3) You now remove your luggage from the bus luggage hold and go through Lao customs. Take another tuk-tuk to your chosen hotel. When travelling southbound, leave central Vientiane at least 3 hours before your train leaves Nong Khai for Bangkok to allow time for border formalities and the various bus/taxi journeys.