A mound is a good idea. It can be dumped on grass. Won't take long for grape roots to gain a foothold in the new soil. Anything that drains well can be used. I'd vote for a good garden soil if you have it. Something that will last and not just decompose and settle in a couple yrs.

Warming the soil some is a good idea. I like the hay in winter bare in summer. Just be sure you aren't making a home for voles or mice. Either could live under fabric or hay and chew on the vines.

We have good luck growing grapes on hills with good drainage here. Something I learned the hard way is that the new plants in Kansas like an old hillside made up with what I consider to not be the best dirt. I grew them in the bottoms and quickly learned they don't like their feet wet. Then I added a bunch of aged cow manure and compost to their ground and found they don't care for that when they are small new plants. Then when I tried growing them in old clay/loam with no fertilizer they loved it. I went back two years later and gave them some old cow manure and they now produce gallons and gallons of excellent fruit every year. What everyone told me to do which was give them plenty of fertility and water just didn't work for me. One day I thought about vineyards and everyone I'd ever seen had grapes growing on a hill made up of primarily poor soil. I went home and duplicated that concept and had good luck ever since.

Let me point out what I'm calling poor soil is very high in minerals. Clay is high mineral soil it's that the minerals are not readily available to plants without the introduction of magnesium etc.