It is 75% of your base pay.
If there is an offset, it is not based on adding percentages. It would work by subtracting the smaller award from the greater award. Hope this is not confusing, but remember, the VA compensation is not taxable, while in the majority of instances the DoD compensation is.
It is not helpful to you to compare the percentages between the systems to arrive at your compensation. The VA compensation is tied to a schedule of steps, so the percentage is not a percentage of a fixed number, it is only a step on a scale. You have to look at the actual dollars you would receive on each side.
But, yes, you can and should file for VA compensation separately from DoD compensation.