Alonso wants a stronger and more complete squad, as there's no guarantee that any player will be injury-free next season. Therefore, Alonso must strive to retain Vazquez, as he is a reliable defensive player, which could also benefit Madrid if other players in the defense are unwell. And if Madrid can't retain Vazquez, they should at least find a more suitable replacement, whether that means bringing in an external player or training the current players.
Building the squad before the new season kicks off is mostly what Alonso seem to be doing at this point, apart from injury, fatigue is another reason these players will need a substitutes at some point because been fatigue can make these players even prone to injury because they will not be at their best when they are fatigued and underperform to the point they could risk been injured on the field of play. Madrid has a good defence even with the absence of vazquez only that it will affect them a while but they can get someone to fit in real quick because they have it.
Getting back up players that will be coming in as substitute and playing in cup games is one thing that is important because he should not be using thesame player as doing so will be putting the players at risk and will not be good for the competitiveness of the club