Vimdiff has just appeared to me when I ran:
hg merge
It has never happened to me before! Is this default now? Because a few days ago I upgraded mercurial.
If Mercurial's "internal merge" is able to figure out what to do it just does it an no interactive merge tool is launched at all. This is probably what has happened in all of your previous merges -- it's pretty good but it only does a merge if it's sure. If it can't figure out how to do the merge then it launches the first merge tool it can find in a long list that ships with Mercurial. That list has always shipped with Mercurial and hasn't changed, though it's possible your upgrade included additional places to look for those tools. It's more likely though you've just never had a merge conflict that internal merge couldn't merge before.
See also: https://www.mercurial-scm.org/wiki/MergeToolConfiguration