You should assume they're using a vi-family editor, since that's what's on-topic for the site. If the specific editor - or sub-family - matters, then ask them to clarify which editor they're using!
We've now disallowed both the vi and vim tagsdisallowed both the vi and vim tags to encourage folks to provide more specific details in their questions. But it's up to all of you, especially those of you writing answers, to ensure this is done consistently - if it becomes impossible to determine which questions - or answers - apply to specific editors, then this will only lead to frustration.