I never said tht VDV 'has the position sewn up' at all, or won't 'be asked to compete for a place'. If you go back and read what I said, he may have been basically told that, given the other CBs at the club, he'll be starting against Brentford, and the place is his to lose. That is completely different to having the position 'sewn up' or not having to fight for it going forward. However that would be completely undermined by us signing another 'expected to start' CB a fortnight later. VDV might have signed on the basis that we weren't planning to sign another top CB this summer. (Incidentally, can you think of any example, ever, where another club has signed 2 'likely to start' £40m+ CBs within a few weeks of each other when they already have one who is deemed competent and expected to stay?).
With regard to your second point, I agree it makes more sense to sign someone to challenge for Romero's place, but I doubt that we would want to lay out that amount of money for 3 players knowing that we will only ever start 2 of them - I just think we would see that money better spent elsewhere. By all means bring in someone else (Tosin for example, if he actually wanted to come...) - but not a player who would absolutely smash our record transfer fee when we have needs elsewhere.
Ultimately I guess it doesn't actually matter what you think, nor indeed what I think. If VDV and Tapsoba think like you, then no problem, let's sign Tapsoba. However if they think like me, and numerous others on here, then that is good reason not to sign Tapsoba now that we have VDV.