Conditionally validate collection/list item properties based on parent model value - asp.net

Question

I have the following view models
public class Step4ViewModel {
[Required(ErrorMessage="Yes/No Required")]
public bool? HaveVehicles { get; set; }
public List<Vehicle> Vehicles { get; set; }
public Step4ViewModel() {
this.Vehicles = new List<Vehicle>();
}
}
public class Vehicle {
public string Index { get; set; }
[DisplayName("Registration/Vin")]
[Required(ErrorMessage="Registration Required")]
[ValidVehicleRegistrationNumber(ErrorMessage = "Invalid Id Number")]
public string Registration { get; set; }
[Required(ErrorMessage="Make Required")]
public string Make { get; set; }
[Required(ErrorMessage="Model Required")]
public string Model { get; set; }
[Required(ErrorMessage="Year")]
public string Year { get; set; }
public Vehicle()
{
this.Index = Guid.NewGuid().ToString();
}
}
The validation works just fine. The problem is that it always works. I only want the validation to fire on vehicles in the collection when the user has indicated that they have vehicles. If HaveVehicles equals false I do not want the validation to fire.
Normally for this sort of validation I would build a custom validator in which I would only return validation errors if HaveVehicles is true. I am unable to do this here because HaveVehicles resides in a parent model which is inaccesible from the within the vehicle object.
One approach I've though of that may work is using a partial view to add/remove the collection from the DOM as null collections aren't validated. If the user selects yes ajax loads a partial view which contains a for loop that iterates over each vehicle and calls another partial view to display it.
Is there a better approach to do this or is something similar to the above my only option?