Last time I flew internationally, I brought a 2015 macbook pro retina with a hard drive full of videos. I only ended up getting 4 of the advertised 9 hours run time running VLC with the brightness turned all the way down. Luckily, there were power outlets hidden under the seats. I don't understand why power outlets aren't the number 1 selling point for airlines that have them.
In the days when I traveled a lot, I used IBM ThinkPad X laptops with expanded (9-cell) interchangeable batteries. You just take a spare charged battery and swap them over.
I once did London -> Chicago -> Seattle and still had about an hour of battery life for the bus from seatac. That wasn't continuous use, but as much use as I needed.
I brought a 2015 macbook pro retina with a hard drive full of videos
Not clear which exact model you have. Some have a discrete AMD GPU which would take more power.
Also it's not clear what format your videos were in. If it's something like DVD rips, then it could have been MPEG-2. I don't think Intel GPUs have any hardware support for playing that back. Also VLC probably doesn't take advantage of any CPU support for playing back video.
You should do a little more research. Your reported performance seems disappointing. You should be able to do much better. For example the current model 13" claims:
Up to 12 hours iTunes movie playback
Notice they don't mention VLC there. :) So you might have to transcode your video into a format that iTunes supports.
It's the low-end 15" model with Intel Iris 6100 integrated graphics, and the files were mostly H.264. I suppose I could have played them in iTunes, but I made a conscious decision to never run it.
