If you can flip the limb clear over using the string moving it up and down its considered vertically unstable.... But like crooked stick said earlier in this thread, you can sill get faily good performance with a floppy limbed bow.
I had a close friend of mine tell me that Long lean wedges help push the working portion of the limb out further, and tip wedges will determine the length of the working portion. But what determines vertical stability has a lot more to do with the limb shape going into the form. an elliptical shape will be much more stable than a true radius shape. It doesn't need to be a flat spot, but less bend shape through the working portion requires more core depth, and that is what gives you better vertical stability. For adding more reflex at the tip, use a tighter radius or arch..... Heavier draw weights are always more stable. More beef..... Arlo