possible itinerary, comments/suggestions welcome

Hi,
I'm going to Laos in the end of February and I would like your comments/suggestions on the itinerary.
I thought of crossing the Thai border from Chiang Khong to Houei Xai, then take the slow boat to Luang Prabang (are there boats in the end of February/beginnign of March?). Stay a few days there, then Vang Vieng (I read it's nice and quiet now).
Should I avoid Vientiane and go straight to Thakhet?
A few days in Thakhet, Savannakhet and Don Deth/Don Khone.
Back to Pakse, crossing the border back to Thailand.
Is it too much for 18 days?
Thanks,
Cristina.

Sounds a good itinerary to me and well-researched, well done.
I would linger longer in the North, also visit Plain of Jars from VV, one can fly from PoJ to VTE. South of VTE is a lot less interesting but 18 days should be plenty.
Certainly linger a full day in VTE - its a delightful little city but not a great deal to do.
If you do spend your time in the North, then the obvious exit is the Friendship Bridge to Nong Khai in TH, then train down to BKK or fly to BKK airport from Udon Thani.
Nong Khai is OK for an overniter.

most people i met said that vientiane was boring ,except for the ecolodge not far. i think giving it a miss considering your short stay is a good idea. i loved luang prabang and everyone i know stayed longer than expected. my most fond time was up to nong khiaw and muong noi. beautiful scenery and good daytime treks. you can stay overnight in a village if you like too.
most people said that vang vieng is indeed quiet and beautiful. my friends stayed quite a few nights there and the scenery is spectacular.
pakse is a go through place for people going to si phan don or to the bolaven plateau. i had to stay for a couple of nights because i didn't make my connection directly to don det by bus when i arrived, and on my return, i stayed because i wanted to do a trek to the waterfalls.
many people raved about the ziplining adventure near pakse and said it was the best thing they did. i was too chicken to go because of the rock climbing portion of the journey.
i went to tad lo by public bus and had a wonderful time.

I have been to Vientiane a few times and always enjoy a few days there. As capital cities go it is not exactly hustle and bustle but it has a charm with the Putaxai, Great Stupa and Wats Sisaket and Si Muang.
A good trip is the #14 bus from the Morning Market (aka Talat Sao) and go to the extraordinary Buddha Park. You may have to change at the Friendship Bridge.
The route also passes the BeerLao brewery and the last few kilometres is along the riverside with views across into Thailand.

Thank you all.
I'll start with the North and see what I can do in 18 days...
I've read contradictory reports on the number of days one gets in the visa, 15 or 30? I'll be crossing the border from Chiang Khong to Houei Xai, and getting my visa there.