I don't think there's a rigorously correct answer; it seems like a matter of author's preference, or a matter for the style guide for the intended publication.
That said, I would choose "V", because I think the quotation marks are the clearest way to emphasize that the "V" is meant as the shape of the letter, not an abbreviation or other usage. Exceptions include widely used terms such as I-beam and U-turn, which don't include quotation marks. (Normally terms would be presented in quotation marks in the context where I wrote them, but I omitted the quotation marks there to emphasize that they are not part of the terms themselves.)
An obvious point is that one should be consistent with whatever usage you choose.
Finally, it seems that the usual outermost level of quotation marks in many countries is single quotes, in contrast to US preference for double quotes.