It is hard for us to respond to your question, because you can use a lot of methods to achieve that (and might depend on other stuff, like: how/where you store your vector, etc)

You can use:
1) spinbox (QDoubleSpinBox most likely because you want double), here you can choose a widget with: one label, one spinbox, and one submit button (and of-course an algorithm to repeat the actions for each element)
2) a spinbox for each element
3) a line edit for each vector (with double values separated by comma, or space or what you like)
4) a textedit (you can use new-line for vector delimitation, and comma or space for values)
5) derive a class from QTableWidget (and make an excel-like program), if you need more computation on the values
... i'm sure that there are a lot more ways to achieve this, imagination is the limit

Think as you are the user... so i ask you: How would you like to enter the input in this application?