You are creating multiple QGroupBoxes in your loop, yet you set each one to have the same QVBoxLayout *vertical = new QVBoxLayout; instance. And you later do page->setLayout(vertical);. Something wrong here!

Well, I guess? I'm trying to group up one label and one textbox..

So this, just via code

grp->setLayout(vertical);

You are creating multiple QGroupBoxes in your loop, yet you set each one to have the same QVBoxLayout *vertical = new QVBoxLayout; instance. And you later do page->setLayout(vertical);. Something wrong here!

(untested pseudocode ;-))

Alright, it does what I wanted, so good job! :)
Just for the beauty I thought about inserting the label and text input in a group box, but I can't get that to work neither.

    int z=0;

    for (int i = 0; i < 3; i++) {
    QVBoxLayout *vertical = new QVBoxLayout;
    for (int y = 0; y < 5; y++) {
        QGroupBox *grp=new QGroupBox();
        QVBoxLayout *vlay=new QVBoxLayout;
        grp->setTitle("Something number: "+QString::number(z));
        vlay->addWidget(new QLabel("Label number: ("+QString::number(z)));
        vlay->addWidget(new QLineEdit);
        vertical->addLayout(vlay);
        grp->setLayout(vertical);
        z++;
      }
      QWidget *page = new QWidget;
      page->setLayout(vertical);
      ui->stck->insertWidget(i, page);
      ui->stck->setCurrentIndex(0);
    }

Sorry for bothering you..

(untested pseudocode ;-))

wdgVector[ss] = new QWidget();
wdgVector[ss]->setLayout(strVector[ss]);

But you also have to initialise strVector.

QVector< QVBoxLayout * > strVector(10); // This only declares a vector containing pointers - you have to create actual objects and put their pointers there. C++ basics.

You never insert any value into strVector.

So how would I do that to achieve the image shown?

strVector[ss]->addLayout(hlay);//<----CRASH

You never insert any value into strVector. The vector is empty so it will always crash when you try to access an element of it.