Altwuerttemberg of course was staunchly protestant, but huge chunks of Swabia belonged to Austria - the territories also known as "the tail feather of the imperial eagle". Not to forget the Reichsstaedte and the itsy-bitsy fiefdoms which also weren't necessarily protestant. Very large monastic holdings especially on the Swabian alb as well. Check out a historical map of the German Southwest in the pre-napoleonic era, before mediatization and secularization - very colorful.
Because of this background, it's probably THE most diverse part of Germany as far as denominations go - frequently one village is protestant and the next one along catholic, and you can just about imagine the clashes you would get in daily life! A real-life story of a farmer woman shopping for a roll of fabric somewhere in the Oberland and disliking the excessive brightness and colorfulness: "No way I'll buy that - that's a Catholic fabric!"
Granted this was some two or three decades ago. A lot of these distinctions have started blurring over the past 15-20 years, and some poor misguided souls have lately even started celebrating carnival in pietist hotspots such as Tuebingen - completely bizarre if you know anything about the background (plus you get perfectly good catholic processions anywhere by driving 20km max).

Back to vesper though - even though there are also distinctively catholic and protestant versions of Swabian (or so I am told by my linguist friends), vesper actually is used in both of them.

(I can't believe this thread is still alive!!)