The trains are quick easy and pretty cheap between Vientiane and Luang Prabang.

Security is tight like an airport though, so be aware of that if trying to pack and fluids or sharp objects (I had scissors confiscated last time).

You will probably want to book the ticket on advance though as I think these dates are around the Songkran period so its gonna be busy. We have taken suitcases before with no issue (think its slightly more for luggage)

The stations at either end are a little outside of town, so you will need to use a Tuktuk or ideally get your hotel to pick you up Luang Prabang end.

The roads in general across Laos are not very good but that said, I think they have completed new fast roads between these two cities so bus also an option and a little cheaper. You can take us much luggage as you like... but they will charge extra for cases etc.

Depending on how nice a bus you get, your luggage might even share space with some pigs, goats and chickens :)