I've been a few times over the last 10 years, always to Vientiane only tho...

There is lots of action in Vientiane, tho I can't comment about elsewhere in Laos as I didn't travel further.
As has been said it's more hidden away due to laws, including that it is illegal to be in a hotel room with a Laos woman that is not your partner/wife.

With that said, the main action is freelancers, the big restaurant bar called Bor Pennyang along the riverfront, or a smaller late night bar back a few streets called SamLo where the ones I always found great action at, but note you have to approach them...

The only brothel style action, with all Viet ladies has been mentioned, think of it as a fishbowl as its close to that.

Massage action abounds, but you have to get away from the tourist riverside areas, when you do you start to get more easily obtainable options.
Tho, it is pretty readily available for everything except sex, when approached and done quietly even in the tourist trap ones.
An approach that works equally as well here is to get a massage and arrange to meet the lady later for hotel room fun.

Once you get 2-3km's back from the riverfront, you will find areas with 5-10 massage shops clustered over a 1-2km area of intersecting roads.
So to find and access them, you need to do your research (ISG has a fair bit, tho maybe dated on locations), then grab a motorbike to get around.

Now the cautionary tale of Laos...
Be very wary of ages in Laos, and yes, semi human trafficking, in the further away massage shops, and chicken shacks (Laos roadside brothels).
Yes, there are many many many small, almost invisible during the day, lit up at night, chicken shack style brothels with 3-5, or 10-15 girls sitting there available.
A fact of life is that any Laos girl of age moves quickly across the border into Thailand and down to Bangkok, Pattaya or wherever, or off to the very very large casino brothel complexes dotting the border across from Thailand in the Golden Triangle area of Myanmar and Laos.

So, what's left in Vientiane?
Well, some quite decent service wise ones there that are older, with the younger ones having most likely been brought in, willingly, or often under some sort of "family arrangement", to make and send money home. It's these ones that you need to be very careful of as they can be often way underage, be they sitting in a roadside brothel chicken shack, or even presented to you in a massage shop for what would be an unskilled and "woeful" massage, then other services.
So, if they look young, don't bother asking for ID, as they likely won't have any, and if they do have some, like Thai ID's, it is that generic looking on the photo that you could never tell whether it was them or not. Also, having seen an ID doesn't help, as it's no protection in Laos to a police shakedown.

My advice is, if it looks young, walk away.
The problem is that Laos girls, look alike with the Isaan girls in Thailand that you'd most often see, as they are ethnically the same.
The scary bit is that many Isaan girls from even 14-15 years old are indistinguishable from late teens 18-19 year olds, and the same for many Laos girls.