I am installing a Vigo shower base on a 2nd floor bath that has been gutted. The base is out of level quite a bit but the existing slope in the base is sufficient to make it drain ok. Can I just install it as is because leveling the rest of the floor would be a night mare? Also I need some help on reinforcing the base. Do I use mortar or cement and how much? The drain sits flush with the subfloor so do I need to put anything around the drain itself? All help is appreciated .

The base needs to sit level, if not, the walls, trim and shower door install will be a nightmare. I use mortar mix. Lay a piece of roofing felt , heavy plastic or Tyvek building wrap on the subfloor first, than add the mortar mix.
It needs to be fairly dry so that it doesn't run out from under the base. Sit the base down in the mortar and wiggle it down. Let the mortar dry overnight before walking on the base.

the base sits on the sub floor so leveling it should not be a problem. shim it and add mortar under it.
of course the drain pipe needs to be sealed to the shower drain fitting. just how you do it depends on the kind of pipe you have and the kind of drain fitting you have.