What are the risks drinking tap water in Laos?

I would personally advise against it, unless you plan on staying there for a long time and are trying to get used to it.

I found a UN report about water quality in Laos. You should read it for yourself to get the details, but here in short:

1. Most water sources/wells seem to pose a health risk.
2. There is little contamination with chemicals and the like, so it's mostly biological pathogens.
3. "Diarrhea, cholera, hepatitis and intestinal parasites" are cited as diseases that can be transmitted via the water, via faecal matter.

I am not so sure how likely those are to occur, but be aware that your immune system not being used to this water, you're more likely to get sick compared to the locals.

If you still want to drink the water, you should find out where the water in the tank comes from. Also ask the locals, while them drinking the water is no guarantee of it being safe, them not drinking it would indicate that there's something really wrong with it.

It's not safe to drink from the tap in those areas, Laos, Thailand, etc. If possible please ask for bottled water. You might not get sick immediately as you will of some parts in India but it's not safe.