Saliba spent 3 season out on loan in Ligue 1 (in his first season he only played 2 games) while van de Ven has 2 season in the Bundesliga (but was only a regular in the last season).From everything I have seen and read about VdV, he isn't on Saliba's level yet. Saliba went on loan for 2 seasons of senior football in a top league after joining them. Now he has 3 seasons of senior football plus 16 games the season before he went there.
Van de Ven has 1 season of senior football in a top league. He's talented but it could take a season or 2.
You sign players of this level with them knowing they have to fight for it and earn it.
Tapsoba would be the only one that maybe comes in expecting to be 1st choice.
If, according Romano (among others), Spurs are after either Tapsoba or van de Ven (and that Spurs are willing to let Dier leave) I would presume they see either one as starters this season and not after 2 seasons.