Could you, please, tell me whether it is mandatory to capitalise lowercase abbreviations in titles (e.g., viz., vs., i.e.), or it's completely wrong, or it's just a matter of preference?
1.2. Kittens Vs. Puppies
2.2. Animals, People, Etc. And, of Course, Mushrooms
It's customary that "short words" - specifically conjunctions, articles and prepositions - are not capitalized in titles (except at the start of the title). I would treat "vs." as a kind of preposition. In any case I would also always leave words like that lowercase. Anything else looks weird. So:
Kittens vs. Puppies
In your second example "and" is one of the words not usually capitalized. It's also weird in other ways - for example you would not normally have another item in a list after "etc." - "Etc." means "and so on" and thus would include all the things you would normally expect to be in the list which would (of course) include mushrooms. You might try:
Animals, People and, of Course, Mushrooms etc.
It doesn't mean exactly the same thing, but if it covers what you want I would use it. The whole thing is a weird title, and if you are not deliberately looking to create a weird title write it another way. Alternatively:
Of Course Mushrooms, and Animals, People etc.
I'm in two minds about the capitalization of "of" and "course". I also think that if you spelled out "etcetera" in full I would probably capitalize it, but not "versus"
