For the VC10, the swept fin does have a purpose. On a T-tail aeroplane, it allows the horizontal tailplane to be mounted far further back. As the engines will be at the back, you need all the moment arm you can get, so it serves a very useful function. I think it fair to say for 707-planform aircraft, the swept fin is not greatly useful. The Comet was quite happy with a stubby, vertical fin.