What Data Annotations need to be shared/different between my Model & ViewModel to keep seperation of concerns?
I'm using localized strings from my views/resources.resx in my Model
  for Display Attributes, does this violate the separation of concerns
  since it's a resource in my views folder intended for views?


I would probably say yes.  You are tying UI concerns in your business models.  Why does your model even care about the text that is used to display it?  It should be more concerned with buiseness rules and performing buiseness operations I think.  Let your UI layer/layers deal with presenting it to the user.


  Do I need a ViewModel for my Model to keep separation of concerns? And
  if so, which Annotations need to go on my model's properties, and
  which need to go on the ViewModel properties?


No I don't think you necessarilly need a viewmodel and there is plenty of debate on this on the internet.  However I personally like the viewmodel approach as it provides the flexibility of adding any UI specified fields I would like without the overhead of my view being tied directly to the buiseness model.  Of course this means also that you now have to have the rules copied twice like you mentioned.  However if you were using jquery validation this would be the case anyway (or so I believe).