I recently received a lovely compliment on Ravelry from someone who admired my design Twist & Shout, but said she hadn’t made it because she couldn’t find a suitable, animal-fibre-free yarn. Well, that sounded like a challenge. (Not that I expect her, or anyone who happens to like the pattern, necessarily to make it; I certainly don’t think I stand any chance of making a tenth of the patterns I like. But yarn substitution is a growing fascination with me, especially since I worked on a few issues of Yarn Forward and Inside Crochet, when I got to suggest alternative choices for the published patterns.)
Any yarn chosen for this jacket needs to be fairly bouncy, and lightweight.* Both are important because there’s just so much knitted fabric involved, and all that ribbing is in danger of stretching out and losing its shape. Sproingy yarn also makes for good cables. Sproing is not easy to come by in plant fibres, though, so you’ll probably need to look for some acrylic or microfibre content, which as a bonus will make it a bit warmer. What immediately sprang to mind was Rowan Calmer – a soft cotton blend that doesn’t behave at all like (heavy, inelastic) cotton – but that’s a DK, and T&S calls for aran. So, to Ravelry yarn search. I was looking for cotton plus acrylic or microfibre, with a specific eye to the yarn construction. Calmer is that rare beast, a true cabled yarn (Z ply on S ply), which is seriously effective at creating bounce, plus loft (lightness!), plus pill-resistance.** As my original messager pointed out, you don’t want to invest that much time and money in a jacket that then pills all over the show…
Turns out there are many, many possibilities. The following are, I think, the most promising; although I haven’t actually used any of them personally. Most of them I’ve never even seen; I am basing my choices on construction and user comments. So, use your own judgment.
Rowan All Seasons Cotton: Reportedly stretchy and light, with good stitch definition. Ticks the boxes. Must admit I haven’t loved it when I’ve looked at balls on the shelf, though; it has a tiny bit of that acrylic feel.
Filtes King Kim: I’ve never even heard of this brand, but the chainette construction and fibre blend look promising.
Plymouth Jeannee Worsted: A good-value yarn, but according to reviews, it still performs.
There, for what it’s worth, you have it. I’d love to hear more ideas, especially if anyone has actually made it with a non-animal yarn and can report back on performance.
_____
* Starmadeshadow, I should admit at this point to being just a leetle worried about the Sublime I recommended for your version. As you know it’s turned out surprisingly weighty. I think it’ll work out, because it is elastic (and so pretty!), but you need to be sure it’s a good fit across the shoulders, and definitely bind off the top of the back and pick stitches up again for the collar – don’t try the crochet cheat I suggest in the pattern.
** Which is why I like Rowan’s pure cashmere yarn – also cabled, making it significantly more durable than most cashmeres.
Woolly thoughts (and fuzzy logic) 
By “cabled” yarn do you mean the type that looks like a shoelace? I saw some Rowan Calmer in London and thought that at the time :).
“Cabled” yarn usually means yarn with multiple 2-ply or sometimes 3-ply strands plied together again. Strictly speaking it should be z-ply on s-ply (there’s another term for yarn that has the plies all going in the same direction, but I can’t remember it right now and my Clara Parkes book is all the way downstairs!) – which is that shoelace look, yes. Although a bit looser and nicer. 😉
The “other” cabled yarn, s-ply on s-ply or whatever, also has a really bouncy, rounded look but is smoother… and it tends to bias.