Southeast Asia forum

The trip is perfectly feasible within 3 weeks. I guess you're not really backpackers and 1 week Laos is enough for you then if you limit yourself to those two places (backpackers will curse me for this).

-fly to Luang Prabang and spend 3-5 days there. depending on interest. it's a beautiful laid back town. if you just want to see sights 3 days is enough. 5 days is better if you want to enjoy also the atmosphere of the place.
- bus to Vientiane is ok. You also have the option of hiring a private car and anything in between. Flight is also possible but depends on time and budget.
- Vientiane. Apart from a few temples there isn't that much to do or see. 2 days should be sufficient.
- fly to Siem Reap . May be there are only flights from Luang Prabang to Siem Reap and then you'll have to do Laos in reverse order. starting with Vientiane.

- in Cambodia you can take bus from Siem Reap to Phnom Penh . Half day rides in decent bus over decent roads.
- from Phnom Penh you'll have different options to get to Mekong delta and Phu Quoc. My prefered route would be to take a bus south to Kep for a night, cross border in Ha Tien and take ferry to Phu Quoc. When leaving Phu Quoc back to Saigon you can make a stop for a day or two in Can tho to see Mekong delta. For other routes check this forum and site

I'd skip Laos.

You've got three weeks of which 6 days are on Phu Quoc, so you've really got two weeks. Allow:
Siem Reap/Angkor 3 days
Phnom Penh: 2 days
Mekong Delta: say 3 days

That leaves you a week to cover the two spots in Laos (without allowing for travel time) so yeah sure, it is possible, but you'll need to fly all the trunk routes and it will be a bit of a mad dash. I'd split the extra week across southern Vietnam and Cambodia, perhaps head to Kep or Kampot... or somewhere like Mui Ne or Da Lat in Vietnam -- that way you've got an excuse for another trip!