I would be surprised if there was significant loss of nutritional value in a soak in water.
The fat soluble vitamins are very unlikely to leach out as they are hydrophobic so vitamin A, D, E, and K are safe.
Most of the metals are going to be bound up in either enzymes or metallothionein, a sulfur rich protein that chelates metals produced in large amounts in the kidney and liver. As far as I understand it, they often use some type of chelating agent like EDTA to remove the metals from proteins as they don't tend to come out easily. This suggests that little of the metals will be lost in the process of a water soak.
The water soluble vitamins do obviously pose a potential risk for loss. The vitamins that are water soluble will most likely have some polar groups like aldehydes or hydroxyls that cause it to like water in the first place. Proteins tend to have a significant amount of these polar groups as well, which leads to hydrogen bonding keeping them closely attached. In order for them to leach out into the water, those bonds would have to be hydrolyzed, which isn't necessarily impossible and may be favorable entropically. However, reactions tend to be significantly slower at lower temperatures, and I would imagine you aren't using hot water to soak the kidneys. The time spent soaking also makes a big difference as this is a diffusion process. If you don't leave them in hot water for a long time I suspect you'll get the vast majority.
Even if you are boiling them, the cells that contain the vitamins would need to be broken, then the vitamins would have to diffuse out of the food into the water. Most likely any considerable loss would be on the outside layers. You could always cook it sous vide and drink the broth, then no loss.