Use the Fluent API to create a one-to-one relationship between two tables' alternate keys

Question

I'm attempting to join two tables in two databases, where the tables have have auto-increment, primary keys and string, alternate keys. The alternate keys in each system's table have been populated to allow data to be compared (joined).

I have two classes:

Unit (system A)

public class Unit
{
    [Key]
    public int ID { get; set; }
    // alternate key
    public string UnitNumber { get; set; }
    ...
    public Vehicle Vehicle { get; set; }
}

Vehicle (system B)

public class Vehicle
{
    [Key]
    public int VehicleSerialNo { get; set; }
    // alternate key
    public string VehicleID { get; set; }
    ...
    public Unit Unit { get; set; }
}

I would like to create a one-to-one relationship between Unit.UnitNumber = Vehicle.VehicleID.

I tried to define this relationship, but have been unsuccessful:

modelBuilder.Entity<Vehicle>()
    .HasAlternateKey("VehicleID");

modelBuilder.Entity<Unit>()
    .HasAlternateKey("UnitNumber");

modelBuilder.Entity<Vehicle>()
    .HasOne<Unit>()
    .WithOne()
    .HasForeignKey("VehicleID");

modelBuilder.Entity<Unit>()
    .HasOne<Vehicle>()
    .WithOne()
    .HasForeignKey("UnitNumber");

What am I missing?