I have one query related to designing WPF using MVVM

Here is the scenario :

1> I have one WPF screen which contains various user controls which are reusable in some other screens too.

2> Can i have separate ViewModel class for each of those user controls , what could be ideal design in this scenario

3> Should i separate my Viewmodel based on individual screen or on UserControls .

4> If i create separate viewmodels based on UserControls how i should integrate it .

Is there any design guidelines around this !!

Urgent Help appreciated ..