Designing an MVC repository using ViewModels

I want to create a repository class to separate out my data logic from my controllers. I am using a ViewModel to represent some data that will be filled with data from different tables.

Here are some questions I have:

1. For a method like GetAll(), do I return an IQueryable<MyViewModel> or IQueryable<Entity>? If I return viewmodels, how do I cope with a GetAll() that pulls thousands of records?
2. Do I create a constructor for my custom ViewModel class that takes the Entity as a parameter to do the mapping? (I'm still unfamiliar with automapper so just need an understanding on how to do this from a design point of view)

Again, my main concern is a method like GetAll() which would pull many records. If I did a foreach loop to translate each Entity into a ViewModel seems like a lot of overhead. My thought was to put a reference inside the custom ViewModel class to the IQueryable<Entity> to access from the collection, and have the ListViewModel just have indexers or something like that which reference the collection property.