There isn't a straightforward answer to this question. Many homes with combustible siding have survived wildfires. Many homes with noncombustible siding have been destroyed in wildfires. Combustible siding can definitely contribute to the loss of a home during a wildfire, but usually other factors also contribute (for example, poor defensible space).
Combustible siding can make your home vulnerable to wildfire in two ways. The first deals with fire spreading up the wall. Flames spreading up the wall will provide a flame-impingement exposure to other components on the wall, including windows, the eave/soffit, and vents. Under this scenario, the combustible siding is transferring the problem to other parts of the house. If the windows, eaves, and vents are fire-resistant (they do not allow fire penetration into the home), then the home will experience considerable damage but won't be destroyed. The second way deals with the fire penetrating through the siding (most likely at a horizontal or vertical lap joint), into the stud cavity, and then into the living space of the home. Under this scenario, the home will probably be lost, particularly if no one is present to defend and suppress the fire. Construction details of the joints can address this fire scenario. Near-home vegetation and near-home storage of combustible materials are always important, but with combustible siding, these items become even more important.
