If you are prepared to invest some time into your search for the Buch family in Vienna, consider delving into the residence records which have been filmed by LDS. When you look at the LDS holdings for Wien/Niederosterreich, click the link to Population, then examine the guidance which the description of each of the holdings provides.

There are three major holdings, i.e., Meldekartei 1895 till 1926 or so, Meldezettel 1930-40, and Meldezettel 1940-47.

To order the right microfilms (with the series on Buch), you would need to familiarize yourself with the phonetic ordering system. Learning the basics of the old German longhand script ("Kurrent") would also help.

The residency records contain lots of data, especially the ones of the 1930/40s which list not only the origins of the residents, but also date and place of the marriage (if applicable).

The same advice applies to other postings on this board, for instance recently the inquiries on Hermann Wechsler
and on Anton & Heinrich Steiner.