For future reference, you may break your journey (or start/stop "short") anywhere en-route with most walk-up tickets (i.e. not "Advance" fares), and always on the return portion of such tickets.
Also (on a point of pedantry), if you really hold a
1st Off Peak Return, then it must have a
Southern Only restriction and as such not valid to Vauxhall/Waterloo

. I suspect you mean a 1st Off Peak
Day Return, which is an entirely different ticket type despite the misleadingly similar name (they used to be Saver and Cheap Day Return).
The
Southern Only day return ticket is around £15 cheaper but there's only 1 train per hour (from Victoria). Journey is about 2 hours so similar to that from VXH. Just thought it might be convenient given your location. 1st class is nothing special on those trains though. The
Any Permitted ticket you'd use from VXH would be valid on the Southern services too, of course.