Well right now Foyth is your presumptive starting right back. So theoretically they’d need one or the other. And just assuming vertonghen stays is useful but who knows if it’s based on reality.
You are the master of assumptions - you should know.
But yes - if Vertonghen leaves as well Spurs would need 2. But considering he keeps on repeating how much he loves playing for Spurs I don't think that is much of a stretch. Right now I am pretty sure the reason he hasn't got a longer contract is down to Spurs - not Vertonghen.
If Foyth ends up as the RB then Aurier/KWP will be sold and replaced with a CD. That would be a cheap deal for another Kevin Wimmer or similar.
Anyway. I don't see the need for big investment next year if the current window ends up being Dybala, Lo Celso, N'Dombele and Sessegnon - other than potentially one expensive CD to replace Alderweireld. Certainly not much in terms of net spend - as I don't expect players like Lucas, Lamela etc. to stay around. Not to mention Wanyama and Dier.