With your list of available forces, you're pretty much hemmed in - watch the plasma weapons, as they'll tend to overheat - we laugh about that in my cousin's IG army all the time. If you aren't going to buy things like Demolishers (hands down one of the most effective tanks the IG has), here's what I'd suggest - Keep the Valkyrie and an embarked Vet squad in reserve, and keep the storm troopers in reserve as well. The reason I'm suggesting the Vets in the Valkyrie is so that they can use the Valkyrie's scout rule to outflank, and come in on a board edge where they're needed most. Otherwise, you're leaving your Vets open to some pretty punishing Tau fire (which I've experienced firsthand as an IG player). I'd advise making at least one squad of Vets a Demolition unit, which will give them meltabombs and a demo-pack. If you can use the Valkyrie to get them close enough to a Tau vehicle, then you should be able to do some real damage. Voxcasters are next to useless, in my experience, and you're probably better off getting a commissar model and throwing it in with the infantry squads, blobbing them up into a large 30+ man unit, and sending them after the Tau troop choices... you'll last a lot longer than they will, particularly with a Ld 10 Commissar in the mix. And, you can give him a power weapon. You don't have to deploy the other Vet squad in the Chimera, either - you can have the Chimera run interference, provide cover for your guys, or just be a mobile bunker.
Either way, you've got some options, and the only thing I'd recommend is swapping out some plasma weapons for meltas... but that's just me. Good luck!
13+ years in the hobby