The routes initially were fundamentally the classic boat train alternatives, once the old route from Victoria was joined a mile or two out. There were various reasons why this range of routes was both available (mostly 19th century history) and line capacity on the main route, extensively 2-tracked with two lengthy tunnels.
I did once go outwards, crossing over to the north lines at Bromley South, then a lengthy signal stop near St Mary Cray which I suspect was asking the driver if they signed via Maidstone, which we eventually continued to, grinding round the curves through Maidstone East, losing our path through the tunnel and thus getting caught on the climbout behind a slow lorry shuttle, and ending up in Paris about an hour late.
It always seemed a bit strange to be running through Vauxhall. I think the initial plan was just to use the northernmost of the Windsor Lines both ways, essentially as a single line, to keep clear of the suburban service, but later they ran conventionally.