yup there are overnight buses between VTE & LPB.
best to get to Ban Naluang bus station in LPB an hour before departure to secure a decent seat.
ordinary bus has taken me 13 hours (~12 hours in daytime).
private vehicles like cars & 4WDs can cram the journey into 9 hours speeding all the way. VIP buses can cram it in 10 hours with crazy driving if there are no breakdowns.
the highway has no lighting for most part, & few crash barriers. personally prefer not to risk my 9 lives with that sort of driving (& cos am short/small enough to fly out of my seat every time they speed around the sharpest bends), so have always opted for the slower journey esp when doing it overnight.
P.S. somtam, maybe sth needs to be checked for the LPB transport info page? both LPB-VTE & LPB-Vang Vieng journey times are listed as the same 6-7 hours...(14-16 hours for LPB-Phongsaly is extremely optimistic too )