"DIT" names are generally found in French-Canadian research. The term "dit"
in French means say or more specifically in regard to surnames,
Usually this indicates that while the ancestor's surname was Vachon, he chose
to go by the other surname. So you could translate it as Robert Vachon called
Pommerleau. This was sometimes done to help differentiate one family from
another that lived in the same area.
Researching such a surname will require that you keep an open mind, looking
for both the original surname and the surname with the dit name. However, if you
find Vachon with two different dit names, it is probable that the two
lineages are different.
For more information on "dit" names, you may want to visit the Francogene.com web
site for a more in-depth explanation.