Well as long as your ply is cdx it's the same structurally as T1-11. The grooves in T1-11 are to 3/8 so that is considered a 3/8 structural sheathing.
You want to snazz it up on the cheap?
Nail some battens on it, that will make it look pretty good just like the old barns, prime it good first. Your lumber yard may still have the old battens laying around...
Oh, by the way you could have used 1/2 that is still permitable as a sheathing in most states, I wouldn't but you can... so don't feel too bad for vinyl it's fine.
Although I like the traditional board and batten for a shed ;) Have a look here just to see what I mean
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