Backfilling a trench

Hi, all! I trenched into my concrete slab to add in a drain pipe so that I could move my laundry room to my basement. My plumber has completed the work and it passed inspection, so now I'm getting ready to backfill the trench. So I'm wondering if anyone can help me understand what I should be backfilling it with. I've got a lot of the dirt that I dug out, so I'd be able to backfill a decent amount with that, but wouldn't be able to backfill it completely with that. I'm also trying to determine whether I need to add a vapor barrier on top before pouring the cement. Lastly, is there a certain concrete that would be best for me to use?

I live in Minnesota and, if it matters, the dirt underneath my slab is more of a sandy wet dirt. It made for very easy trenching as it dug out well, while at the same time, the sides of the trench did not collapse in at all. So my three questions are:

• What do I use to backfill the trench?
• Do I need a vapor barrier?
• What type of concrete should I use?

Appreciate any help anyone can give!