Here's how I packed food for an overnight trip on the train, it was two whole days and one night.
I had a little soft sided cooler, and I made fresh juice in my juicer and froze it in water bottles, carrot/apple, and grapefruit. I think I took three of each.
I put the frozen juice bottles in there and they served the same purpose as an ice pack. I probably had one small ice pack too.
I had some sliced cheese in little bags, and some carrot sticks and an apple. Wish I had brought more apples.
I put the cooler in another tote bag and had some rice crackers, corn chips, and trail mix in there.
I did very well with this. It was enough and the juice stayed frozen and/or cold until the very end.
It was kind of heavy to carry at first though.
My diet is absurd, I am a vegetarian and can't eat any wheat at all and pretty much no other grains either except rice. Restaurant menus don't work very well for me
but what can you do. I don't really want to pay for it anyway so that's okay
If I take a sleeper trip dining will be an adventure. All I really want to eat is salad anyway. Maybe that would work out okay. You are brave to be vegan. I lean toward wishing I was on a raw food diet to some extent. I guess people who do that, do travel.
Those little soft sided coolers are handy, and when you are done with them they collapse. Mine was only 8.00 and I have used it for picnics a lot.