Is there a reason to not dice vacuum pack vegetables and keep them in the fridge for å few days before cooking them sous vide?
As far as food safety is concerned there is no issue. A few days at fridge temps won't grow anything scary.
Food quality may be a concern. Vegetables want to be able to breathe. Some will handle being sealed worse than others. The problem is having the plastic in contact with the vegetables will hold condensation against them and promote spoiling. Luckily, the kind of firm veggies that can benefit from sous vide will also handle being sealed for a little bit.
Just don't hold them for more than a couple days and watch for mold.