can you not get towbars that can detach when you're not using them? So get one of those fitted, keep it detached when it's not in use but when you need to transport bikes, fit it and get a towbar mounted device.
To be frank though, if you can afford a 4×4 and a vantage, I'm sure you can also afford a car to go mountain biking in. I'd never in a million years take a vantage mountain biking, it'd get covered in mud, scratched, stink of wet clothes. I'd just buy a cheap transit.