I think calling her Countess of the Vinzgau would be a mistake, in the absence of a primary source calling her that.
The reasoning seems to be that her father Gerold was Count in the Vinzgau, and the Germans practiced partible inheritance, so even though she was not an heiress she would have the same titles her brothers had.
There are a number of problems with that.
First, the modern spelling is Vinschgau, older spelling Vintschgau, not Vinzgau. Vinzgouw looks like it would be a Dutch spelling, but if I remember correctly it's taken from a Latin charter. I could be wrong about that. Further, Vinschgau is actually in Tirol, not Swabia.
Second, there is a question about whether Hildegard's father was really Count in Vinschgau. MedLands says only that he was Count in Kraichgau. German Wikipedia says he was Count in Kraichgau and Anglachgau, and had estates in Kraichgau, Anglachgau , Wormsgau, Lobdengau, and Ufgau (around Worms and Heidelberg).
http://fmg.ac/Projects/MedLands/SWABIAN%20NOBILITY.htm#_Toc359920225
http://de.wikipedia.org/wiki/Gerold_von_Anglachgau
Private User and I had an extended discussion about Gerold a few years ago. I don't remember the outcome, but he gives his opinion on the profile for Gerold, count in Kraichgau and Anglachgau: "Some English sources still insist on [calling him] Gerold of Vinzgouw, though he held no land in any such named territory. His area of origin is unknown - ethnicity is Frankish." Ben also says Gerold was Count in Westbaar, and possibly Alsace.
French Wikipedia adds that Gerold was Count in Baar. And further that his possessions on the Middle Rhine suggest he might have been a Frank installed in Alemannia after an uprising of the nobility there.
http://fr.wikipedia.org/wiki/G%C3%A9rold_Ier_de_Vintzgau
Third, at this period Counts were Imperial officers. The title was not yet hereditary and it was not associated with a defined area. We make the distinction by saying these counts were Counts in ... rather than Counts of ...
Hildegard would not have shared in her father's title, nor was there a title for her to inherit. Her brothers didn't even share the title.