As always, you make me think about a different point of view.
We have already discussed the Royal Lao bloodline, so I will set that aside, without comment.
As to who took land, you have not mentioned Vietnam, China, or Cambodia. For example, Dien Bien Phu was inside Lan Xang, but you have not spoken about the Vietnamese government stealing from Laos.
Even Dalat, Vietnam, the famous market town, was probably inside Lan Xang. Nevertheless, the Mekong river was the boundary line decided on by the Lao Kings (Luang Prabang / Vientiane) and Sukothai, as Ayuttaya grew to power.
About a hundred and 20 years ago, the French government requested special permission from Siam to use the Mekong river. The King graciously gave them permission to trade, but instead, the French sent armies from Saigon to take control of Laos.
By using your reasoning, based on old agreements, Laos has a province that belongs to Thailand and as Cambodia has nine or so provinces of old Siam.
You say the Vietnamese and Lao governments are the same. However, most of the world sees the Vietnamese government make a decision and the Lao government jumps quickly to comply.
To the world, Laos looks like a puppet.
Vietnam, China and and Cambodia stole land from Laos, but these land are isolated and not a populated area for Lao people so we are not fussed about it. For example, Dien Bien Phu that you mentioned is near the north eastern part of Laos where there are mountains, jungles and Hmong villages. Lao people don't live near the Vietnamese border so they can take more land if they want.
The majority of Lao people live along the Mekong River and this region is considered rich land. Thailand took rich land from Laos where Lao people have lived for centuries and that's the difference.
The Vietnamese and Lao governments are the same in that we are a Communist state. Vietnam doesn't make decisions for us, they may give some guidance and advice but the same as what China and Russia do for us.
People forget that Laos is only a small country so it's difficult for our voice to be heard. If we were a more bigger and powerful country than China, Russia and Vietnam then we would have most of the say.