It's not just aviation, it's standard in English.
The slash has become standard in several abbreviations. Generally, it is used to mark two-letter initialisms such as A/C (short for "air conditioner"), w/o ("without"), b/w ("black and white" or, less often, "between"), w/e ("whatever" or, less often, "weekend" or "week ending"), i/o ("input/output"), r/w ("read/write"), and n/a ("not applicable"). Other initialisms employing the slash include w/ ("with") and w/r/t ("with regard to"). Such slashed abbreviations are somewhat more common in British English and were more common around the Second World War (as with "S/E" to mean "single-engined"). The abbreviation 24/7 (denoting 24 hours a day, 7 days a week) describes a business that is always open or unceasing activity.
(Slash (punctuation) § Abbreviation, Wikipedia)
Note that there is a difference between abbreviation and acronym, though that line in recent usage is blurred.
Boeing uses VS for the 777, and V/S for the 737. Language is more about what's more common, preferred style, and/or trends (e-mail becoming email). For example, knowing when to use a hyphen, space, or no-space, for compound words requires checking a dictionary. There aren't fixed unchanging rules to follow really in language.
There are some questions on the topic on English Language & Usage Stack Exchange.