Vietnam government changed "old visa exemption rule" to "new visa exemption rule" (have to wait 30 days passing for re-entering into Vietnam with no visa) several years ago.

Before Vietnam government changed the rule as above, i repeated to exit & re-enter into 3 countries (Vietnam & Laos & China) in a long trip to 3 countries getting a new stamp for "15 days staying with no visa"=15 + 15 + --- . But now it's becoming impossible for me to re-enter into Vietnam soon just after exiting Vietnam under new visa exemption rule.

But other 2 countries(Laos & China) have not changed visa exemption rule yet, at least, till today so if you exit Laos and enter into Vietnam then re-enter into Laos , you can get a new stamp of 15 days staying with no visa or 30 days staying with new visa at Lao border(VOA).

Or do I have to wait a certain amount of time to re-enter Laos?

---in case you go to Vietnam, i reccommend you to stay(or trtavel ) in Vietnam for several days before returning to Laos for reducing a conciousness of waste of money .

"Which border crossing point is the best for you" depends on your travel schedule in both countries and where you are staying in Laos now.
①Pang Hok(Laos)/Tay Trang(VN) border---between Muang Khua and Dien Bien Phu(5 hours only between 2 by direct bus)
②Nam Phao/Cao Treo(VN) border---between Vientiane and Ha Noi (distance between 2 is very far)
③Nam Xoi/Na Meo border---between Sam Neua and Thanh Hoa, if you can stay at Viengxai village near Sam Neua, distance from Vieng Xai village to Lao border(Nam Xoi) is 54km only/ Sam Neua to Lao border is 82km.
④Nam Khan/Nam Can(VN) border---between Luang Prabang/Phonsavan and inh (distance between Phonsavan and Lao border is very far)

⑤Na Phao/Cha Lo(VN) border---Thakhek(in southern Laos) and Dong Hoi near Hue(direct bus via Na Phao/Cha Lo border is not daily service)
⑥Densavanh/Lao Bao border---between Vientiane・Savannakhet and Hue(distance between VTE and Lao border is very far)
⑦Phou Keau/Bo Y border ---between Pakse・Attapeau(119km from Attapeau to Lao border) and Ngoc Hoi(19km from Bo Y border)・Kon Tum(60km from Bo Y border） in Central highlands/there are several direct buses between Laos and Vietnam

I think "①(=in case you are staying at northgern Laos now) or ⑦(=in case you are staying at southern Laos now) + staying at Vietnam side for several days is better.
---①＆⑦ are easy to access to Lao border and border crossing process is smooth and finding a transport for returning to Laos is not so difficult(you need to check it at Vietnam side asking to Vietnamese for yourself if you choose this route)

If Garman tourists can be applied "visa exemption rule" for entering into Thailand and you are staying at Pakse or Vietntiane now , "Vientiane/Nong Khai border" from Vientiane or "Vang Tao(Laos)/Chong Mek(Thai)" border from Pakse spending half day only is another better option.
---walking around VN immigration building(house) or Thai Immigration building(house) for few mins only or take a coffee break only then return to Laos is OK.

(I'm not good at English so maybe difficult to read and understand my replying ut all my comments are based on my actual travelings and experiences)

(by jp_geckozy)

We visited Chiang Mai this year from 8th Mar-5th Apr and had to stay inside a fair amount. If it's similar smog for you, you probably won't want to do any hiking at lower levels because it's so dangerous. We did visit Doi Inthanon during our stay and this was the view showing just how bad the smog was at the time (hopefully this link works!):

https://notbritsabroad.com/wp-content/uploads/2019/06/IMG_20190329_102453-640x480.jpg

We did manage to do a few outdoor activities like Doi Int and ziplining but we wouldn't go out in the city without face masks on. Actually, we ended up spending a lot of time in the cinema and indoor restaurants. That being said, CM is a really great place to visit and there's loads to do there. Just be safe when you visit if it's smoggy. I don't regret going at all and loved it regardless of the smog but it would've been nice to walk around more.

I would probably switch the whole trip - fly home - Bangkok - Luang Prabang, travel overland / over river to Thailand and then make your way down. That way you can start in LP, which is much more relaxed than Bangkok, you can book your BKK-LPQ flight ahead of time, and you are in general more flexible to change you itinerary and/or how long you spend where whilst traveling.

Huay Xai is a popular way to enter/exit Laos, not a destination as such, and is north of Luang phabang. As your flights are booked you will be looking at some long bus journeys, and you will have to backtrack to Vientiane. If planning to travel the entire length of Laos, the logical way to travel is via Thailand, which enables you to travel from Huay Xai in the north, to Si Phan don in the south, or vice versa, without backtracking. If you want to see the south, you will need to cut some destinations from the north, Huay Xai for one. If you want river boat trips look at Nong khiaw to Muang khua. Laos is not a country to rush around.

Having thought about this a bit more, I am wondering how important the burn off is and whether there may in part be another explanation to the dust haze.

Having cycled nearly 1,000 from Mae Sot to Chiang Saeng, I only recall seeing 2 burns and both were small. There were other areas that had already been burnt, but it wasn't widespread. What I have seen however, is the disappearance of the rubber plantations that covered much of the countryside. Yesterday there were fields as far as the eye could see given over to pineapples and to a lesser extent, cassava. I'm sure these would have been rubber trees 15 years ago.

All the open fields are dry and the slightest breeze will pick up the dust from the surface. In contrast a rubber plantation was a more humid environment and the floor was usually covered in grass or sometimes rotting leaves.

Maybe it is this secondary wave of 'deforestation' that is responsible for the underlying problem.

I agree with the posts above recommending that you carry USD for maximum versatility or simply use ATMs. Changing CAD to THB just so that you can then buy LAK in Lao is madness, and it is twice as mad to buy THB in Canada.

Carry 100 to 200 USD for emergencies, but only use it for emergencies and keep it separate and secure.

Would be useful to know the time of year that you will be travelling.

This is what I will be doing in August / September this year, and may suit you too (I have been in all these areas several times before):

Fly to Hanoi, stay 2 nights
Overnight train to Lao Cai and bus to Bac Ha, Stay in Bac for 3-4 nights, which will include the weekend ethnic markets, and exploring by motor scooter on other days.
Drive from Bac Ha to Ha Giang via Xin Man over two days, with private car and driver.
Hire motor scooter for 7-8 days, loosely following the "Ha Giang Loop" (search online for more info) and probably going as far as Bao Lac
Bus from Ha Giang via Bao Lac to Cao Bang
Bus from Cao Bang to Lang Son
Bus or Train Lang Son to Hanoi
Last two nights in Hanoi
That is about 18-19 days in north Vietnam

Fly Hanoi to Can Tho (in the Mekong Delta), same day bus Can tho to Chau Doc
2-3 nights in Chau Doc
Bus Chau Doc to Ha Tien, border crossing to Kampot Cambodia
3-4 nights in Kampot
Kampot to Phnom Penh to Kompong Cham by bus
3-4 nights in Kompong Cham
Bus to Siem Reap for last 2-3 nights
This Mekong Delta / Cambodia section about 14 days

All of these places are highly recommended!