Luang Prabang to Vientiane

It depends on your needs, the quickest way is a flight from LP to VTE, costs maybe $150, but I enjoyed the old rattletrap buses with a night or three at Vang Vieng at about the halfway mark. If you have more time, then sidetrip of to Phonsavan for a day or three with the Plain of Jars.

Best way: luxury vehicle with soft leather seats, cushions, a drinks fridge, great music, and lots of leg room.

Realistic way: bus.

If you are short of time, flying is the best only 45 minutes. If you have time, hire private minivan.

Neither best way is local bus mix with the baggage and also VIP bus windows dirty and as scenery through Pou Khuom is fantastic for photo shots and in mionivan yuo cant see over the hedges also with local bus the windows slide open for that photo opportunity .
Also despite past bad press stopping over in Vang Vienne is worth it arrive from early bus Luam P around 3pm sunset on river and next am tubing (the crazies swings and Bob Marley blasting out have been removed) so nice floating past the karsts then afternoon reclining Bhudda caves and next am onto Vientianne and out
Vietnianne is around 4 hrs from Vang V and Vang V 6-8 hrs from Luam P

we've done both.

If you've got time, go by bus, take long rides by cycle in the countryside in V V,

great journey.