1-Shot/Chromatic or Ronan bulletin paints and roller it on. Some use foam rollers while others use nap. Make sure you clean the board and get all the dirt and grime off the board before you start. Three coats should do it. Some will say you need to primer first, while others will say to get pre-primed. However, if you're going to get half it pre-painted, you might as well get RTG panels. That's panels totally prepainted from the factory. You just have to touch up the edges. No muss, no mess and you're ready to apply vinyl immediately.
It depends on how big the board is. I've done 4x8's with 24" vinyl. It leaves a seam down the center but I've never had any complaints from clients. If it is double faced you have to be really careful because the vinyl will damage a lot easier than paint. The nice thing about vinyl is that you can decorate the piece imediately. I've wrapped the edges of MDO with vinyl, cutting it in 2" stripes and dividing the margin on both sides. I've only done that on single face displays. I've kept tabs on the jobs and they have lasted for years so far. It's a lot easier than painting the edges. I prefer covering MDO with vinyl to painting, its cleaner and the cost is about the same. Just be careful to not have any dirt on the board. Everything shows.
Not only do I always, always, always paint MDO.........I always purchase it 2-sided and enamel coated. Then I scuff it with a scotchbrite and finish coat it again with an oil based enamel.
I need to say this: If you do choose to vinyl it.....make sure it's all enamel coated finished first. Believe it or not, I've seen some guys just put vinyl on MDO just raw or just primered. And obviously (to most) it doesn't last outdoors very long.