If a library in VIAF makes its own authority records accessible through VIAF (like BnF, DNB and a few others) I tend to cite those directly, rather than use the form "So-and-so in VIAF." The two most usual cases when I would cite VIAF "straight" are:

1) When there is great discrepancy in the forms used, as for instance, VIAF ID: 47830304, where two widely different death dates are given for the same person, 1499? and 1521. (The German National Library date "1521" is wrong, and I have contacted one of the sources they cite with an explanation of how the error occurred; a correction is promised.)

2) When there are multiple clusters in VIAF for the same person. In this case I list the various VIAF ID numbers as if they were pages in a print reference work, and give the various forms for each number in parentheses. VIAF ID: 47830304 is also an example of this; it is one of three different clusters in VIAF for Henricus/Heinrich/Henri Mayer, an early printer of German origin, active in Toulouse. There doesn't seem to be a good channel to provide feedback to VIAF about this sort of thing, so I just note it in my record.

By the way, if you're citing the various forms used by different libraries in VIAF, there is an easy way to collect all the data: Select and copy the data in the list at the top of the record, little flags and all. Either copy and paste it line by line into your 670 field, or copy it all and paste it into a text file, or a word processing file using the "text only" option. All the little flags will miraculously turn into library names; you'll only have to get them all in one line and adjust the punctuation. Thus (straight from my clipboard to this message):

Mayer, Henri ‎  ISNI 
Mayer, Henricus, -1499? ‎  Library of Congress/NACO 
Mayer, Henricus (14..-1521). ‎  NUKAT Center of Warsaw University Library 
Mayer, Heinrich -1521 ‎  German National Library 

(This is with my setup, using the Chrome browser. Your results may vary.)

Kathie Coblentz | The New York Public Library
Rare Materials Cataloger
Special Collections/Special Formats Processing
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My opinions, not NYPL's