I haven't done it by bus but know from people who have, that according to the schedule, it's 40 hours from Vientiane to Kunming, but often takes 48 hours. Thai, Lao and Vietnamese buses do just fine driving overnight - they usually have 2 drivers available per long-distance bus route though, so this rule probably only applies to Chinese drivers. Although good from a safety point of view, it's inconvenient if you're a traveler because it greatly extends your journey. A better option is to do the trip in stages (and isn't that kind of the point anyway, if you're going to Laos you want to see something along the way?)

Although if you can't stomach the terribly winding roads, there's always a Jinghong-Luang Prabang flight operated by Lao Airlines. Or of course Kunming-Luang Prabang and Kunming-Vientiane.

Personally I'd recommend flying Kunming-Jinghong, then going to Luang Nam Tha by international bus, then Luang Nam Tha - Luang Prabang by van, finally Luang Prabang to Vientiane by plane (or if you want to make a stopover in Vang Vieng, need to go by van).

Carrefour exited the Thai market years ago, having had their business purchased by local supermarket giant Big C.

Surprised Carrefour lasted this long in China. Before long, Walmart will be sold to a local firm too.

Yes there are, already since many years but they take forever. Unlike in Thailand and Vietnam, where only short breaks are taken, Chinese bus drivers like to take 6 hour breaks at the border or something, meaning a journey from Vientiane to Kunming often takes a bone jarring 40-48 hours. A bit too long to be hauled up on a bus I think. Given you can fly from Kunming to Jinghong for under 400 Kuai, if you can at least do that part of the journey by air, it becomes more manageable.

A few years ago I was regularly wiring money out of China from the BOC without needing any documentation, but I could only convert the equivalent of USD 500 of Yuan into foreign currency everyday, meaning in order to transfer USD 2000 out of the country, would require 4 trips to the bank, once a day over 4 days.

Is this method still available, or have they now made it more difficult?

Chinese trucks can't cross the Thai-Lao friendship bridge. They've never been able to, either (unlike private cars, which now need a tour since 2016). However, goods that are being transferred between Thailand and China usually on Thai-registered trucks as far as Boten on the Lao-China border cross regularly. For the average person, it doesn't make a difference but there is a change of trucks that takes place at the Lao-China border and there are companies that can offer such a service without you having to worry about the finer details like the country of registration of the truck.