Whether to add a comma after viz.—or, for that matter, after e.g. or i.e.—is a style question that different style guides answer differently. For example, The Oxford Guide to Style (2002) has this:
Do not confuse 'e.g.' (exempli gratia), meaning 'for example', with 'i.e.' (id est), meaning 'that is'. Compare hand tools, e.g. hammer and screwdriver with hand tools, i.e. those able to be held in the user's hands. Print both in lower-case roman, with two points and no spaces, and preceded by a comma. In OUP style 'e.g.' and i.e.' are not followed by commas to avoid double punctuation; commas are often used in U.S. practice.
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Take care to distinguish 'i.e.' from the rarer 'viz.' (videlicet, namely). Formerly, some writers used 'i.e.' to supply a definition or paraphrase, and 'viz.' to introduce a list of items. However, it is OUP's preference either to replace 'viz.' with 'namely', or to prefer 'i.e.' in every case.
Consistent with that advice, The Oxford Dictionary for Writers and Editors (2000) has this entry for viz.:
viz. videlicet (namely) (not ital., comma before), but use namely
But The Chicago Manual of Style, fifteenth Edition (2003) endorses the opposite approach in a section titled "THAT IS, NAMELY, FOR EXAMPLE, OR, AND SIMILAR EXPRESSIONS":
6.44 Commas customary. Expressions of the that is type are usually followed by a comma. They may be prceded by a comma, an em dash, or a semicolon; or the entire phrase they introduce may be enclosed in [parentheses or em dashes. When or is used in the sense of "in other words," it is preceded by a comma.
[Relevant example:] Bones from various small animals (e.g., a squirrel, a cat, a pigeon, a muskrat) were found in the doctor's cabinet.
Note that "e.g." and "i.e." are not italicized.
Since viz. is the Latin abbreviation equivalent of namely, the "comma customary" guideline presumably applies to it. Elsewhere (at 15.45) Chicago includes viz. on its lengthy list of scholarly abbreviations, about which it says, "Note that Latin abbreviations are normally set in in roman."
So Oxford and Chicago disagree about whether viz. (and i.e. and e.g.) should be followed by a comma. Chicago considers the comma "customary," but Oxford bans it. If you are free to choose your own style, I'd say that the factors to weigh are whether "double punctuation" bothers you (as it does OUP) and whether "customary [U.S.] practice" is something you care about. Another practical question you might ask yourself is whether you would be inclined to put a comma after namely if you decided to use it in place of viz. in the particular instance you're dealing with.
