Laos - north or south or both ?

Hi

at first i wanted to explore northern laos and then go to vietnam, but now i
want to see 4000 islands in southern laos ( is it worth seeing ?), so
this is my plan : from bangkok to Pakse and south to Muang Khong and
waterfalls then back north to Bolevan Plateau and Vientiane where i want to
extend my visa for another 14 days which i spend i northern laos and after
that i.m going to vietnam.

what do you thing about that plan ?

bodzio
thx