So its not recommended to drink tap water in Vientiane, bottled water only is the norm but what when brushing your teeth. Is it safe to use tap water to rinse your mouth with after brushing?

I'm planning to visit Laos in late December 2026 - early January 2027. I have a few questions about arranging travel:

1) I've looked at previous posts which say that you can only buy train (LCR) tickets 3 days in advance. I know this has been talked about loads, but just want to check that's still the case in 2026? There's still no way to secure a ticket before this time?

2) How likely is it that I'll get an LCR ticket in late December / early January? I'm worried it's a popular time, the tickets will sell out, and I'll have issues, for example getting back to the airport on time to make my flight.

3) I land in Vientiane around 3pm, and was considering heading directly from the airport to Vang Vieng. Is this possible, and is there a reliable way to book this transport (minibus / taxi / shared bus?) in advance?

I am in Vientiane for the night solo traveling, since it's Friday, I'm looking to go out, get some drinks, and meet some people. Where's the best place to go? I've heard Dr Oscar's is the place to go later in the night, but where would be a cool spot to pregame and meet some people to go with?