Transit at Bangkok on two different tickets

If your flight arrives in time, 2.5 hours is well enough. Suvarnabhumi airport is very well organized, immigration procedure and baggage claiming should not take longer than 45 minutes, from the baggage claim to check-in its not longer then 10 minutes and Lao airlines stuff is quite relaxed so don't worry if you won't make it within 2 hours before departure to the checkin counter..

if your flight has a serious delay (1/2h or more), i would recommend the following : Make sure you have filled your thai arrival card (you get it on the plane), hurry out of the plane and to immigration, ask one of the friendly stuff in front of the immigration that you are short on time with your connection flight, they will let you pass the queue. You still have to wait for your baggage so, but if your are not travelling alone, one of your party can rush to check-in counter and ask them to wait for you and your baggage... It will work, in Laos it always works someway.. :-)

As your flights are on separate tickets, you need to have:

• Deplaned (can take a while on a large plane)
• Reached immigration
• Queued at immigration
• Gone through passport control
• Reclaimed your bags (possibly waited for them first)
• Gone through customs
• Reached the departures area
• Found the check-in area for your second flight
• Checked in/dropped your bags before the check-in deadline

In most cases, the check-in deadline is around an hour before scheduled departure time. However Lao Airlines state that their deadline is 2 hours before departure for international flights.

I'd say you have about 0 chance of making it.

Remember that if you don't check-in in time, you will be considered a no show, and for most fares (especially the cheaper ones), that means your ticket (including all subsequent segments on the same ticket) will be automatically cancelled. You will be on the hook to book and pay for a new ticket, which, at the last minute, can be very expensive, and will probably imply an overnight stay.

2 hours is pretty unusual, so you may want to check with them if that indeed applies to all their flights.

Even if the deadline is shorter, it's still a pretty tight connection if the incoming flight is on time and everything goes smoothly. If there are any delays or complications, it would quickly become impossible.

You should look at alternative arrangements.

Will I need to go through Thai immigrations to collect my luggage? As a Dutch citizen it says here http://www.consular.go.th/main/contents/filemanager/VISA/Visa%20on%20Arrival/VOA.pdf that I do not need a visa.

While you don't need a visa, you still need to pass immigration to enter the country. That means having to queue up with most other foreign nationals. There may be a few nationalities than can go through automated immigration, but the Dutch aren't among them (yet).

Note that you have to pass immigration twice, once on your way into Thailand and once leaving Thailand. You'll also have to go through a security check where you have to take off your shoes and have your carry-on scanned. Not a big deal, but it's another queue.

Does this mean 'faster through immigration'?

Immigration will be fast for you, provided you fill in correctly your arrival and departure cards on the plane. While there are reports of those cards being phased out, they weren't last December when I was there. But like I said, you're in a queue, and if the people in front of you take longer you will still have to wait.

All in all, I would strongly advise against such a short connection. While you might make it if all goes well, any hiccups could cause you to miss your flight.

If you've already booked your flights, you may want to look into priority queues (e.g. this one, though I can't vouch for them) for immigration (specifically when arriving in BKK, as you want to drop off your bags for the second flight before check-in closes).

Note that these priority passes are nice, but they won't help if your first flight arrives late and it's likely you'll still have to wait for your bags to arrive.