You've asked a bit of a subjective question here without much possibility of a definitive answer. However, I'm going to give it a try.
In my opinion, I would always manipulate data in the view model, for two main reasons:
- Data manipulation = business rules and that belongs in the view model and not the view
- You have access to every property of the data item(s) and every method in .NET in the view model, but access to very little in a
Trigger
.
I hope that helps in some small way.