What region did my ancestors live in?

As we know for sure they had come to the Beresan district, Odessa region.

 It's highlighted with lightskyblue:

http://library.ndsu.edu/grhc/articles/magazines/articles/ndrvossler.html

In the USA a regional interest group concerning this region is to be found under: http://www.grhs.org/chapters/bdo/ 

and partly on the website Black Sea German Research: http://www.blackseagr.org/learn_odessa.html

or at the Odessa Office for Foreign Settlers in Southern Russia: http://www.archives.gov.ua/Eng/guides-Odessa.php 

Here is another map of those villages:

http://www.auswanderung-rlp.de/auswanderung-nachosteuropa/russland/auswanderung-ans-schwarze-meer.html