I'm planning on going to Vietiane in Laos, but I wanted to cross over land from Cambodia. Are there direct bus connections from Siem Reap to Vientiane, and if so, how long does the journey take? I have heard conflicting reports - some say yes, some say there's no way around it but to double back to Phnom Penh and catch a bus from there to Vientiane, which doesn't make sense at all to me. Or, if not a direct bus from Siem Reap, then is it possible to get to Stung Treng checkpoint by local bus, then connect to Vientiane from there? (If not bus, boats / ferries are acceptable too.)
I have also heard that even though the official Lao immigration site says visa-on-arrival is available at land border checkpoints, they have stopped issuing them at Stung Treng and require visitors to get a visa beforehand. Is that true?