I tore out my front porch slab so I can waterproof the foundation. It was also sinking and since it has a column to support an overhang, that's a problem. My question is what should I do about where it needs to come up to a level higher than the bottom of the vinyl siding? In an ideal world, the slab for the porch would be against the block foundation of the house, staying well below the siding. The level of the slab also puts it above the wooden bottom plate of the house. In the picture you can see by the dark line on the siding where the main level of the slab was (there is also a step up to the front door). It also looks like no forms were used by the house, the concrete was just poured against the siding. How would I do better?
You can pour a new slab up to the level of the foundation, not higher. In this case, you could basically fabricate a new concrete patio that's below the level of the previous one and build some steps leading down from the door to the patio.
You could also forget about making a concrete patio and build a wooden (or composite decking) patio at the current level of the door.
In general, I would recommend against pouring concrete right up against the foundation, because it provides hidden termite entry points and makes it very difficult for you or future owners to insulate the exterior of the foundation later (if this is an appropriate foundation insulation and hasn't already been done). Also, you have to get the slope away from the house perfect or else you're redirecting water into your foundation.