While pouring our porch the concrete splattered onto the vinyl siding.How can we remove it without damage?

We recently added a concrete porch across the front of our house.While pouring the concrete some of the concrete splashed onto the gray vinyl siding. I tried a pressure washer set at a low pressure and it took a small amount off. How can I remove the concrete without damaging my vinyl siding?

I’m pouring a cement pad that’s 11 1/2 ft x 11 1/2. What is the minimum depth I can safely get away with?

Also, I’m pouring this slab inside a dirt-floor barn, that already has a concrete foundation around the sides. Can I pour the new concrete right butt-up to the old concrete foundation? or do I need to use some kind of spacers? If so what kind? wooden? how big? [...]