If a substantial portion of the mass of a planet like Venus or Mercury were mined, it would have an effect on their orbits, albeit a relatively small one.
According to Newton's law of universal gravitation, the gravitational force between two objects is directly proportional to their masses and inversely proportional to the square of the distance between them. When a significant amount of mass is removed from a planet, its overall mass decreases, which in turn affects the gravitational force it exerts on other objects, including itself.
In the case of Venus or Mercury, if a large amount of their mass (25% or more) were mined, the gravitational force they exert on other objects, such as the Sun or other planets, would decrease. As a result, their orbits would be slightly altered.
However, it's important to note that the overall mass of these planets is significantly larger than any foreseeable mining operations. Even if a substantial portion of their mass were somehow removed, the effect on their orbits would likely be relatively small compared to their initial orbital parameters.
Additionally, other factors such as the gravitational influence of other planets in the solar system would continue to affect their orbits. Planetary orbits are complex and depend on a multitude of factors, including the mass distribution within the solar system.
In summary, while mining a significant portion of the mass of Venus or Mercury would have some effect on their orbits, the magnitude of this effect would likely be small compared to the initial orbital characteristics of these planets.