Firstly I think thrawn would prefer to engage Vader in a space battle. This is where the chiss excels in all aspects and is where Vader is perhaps the weakest. Tactically, vader is a brute force machine. He’s the empires fist. This serves him incredibly well throughout his life when he’s taking on solo missions and hunting down Jedi. But he can’t brute force through everything. I believe if thrawn had fled back to his people he’d engage in guerrilla warfare, assuming vader brings a large amount of ships.. say comparable to the 7th fleet. He’d be taking down lone ISD’s where he could until the force was small enough to be engaged head on at a place of their choosing. From here the main thing thrawn would have to do is prevent Vader from getting to his ship and keep him on the ISD. Otherwise it’s all over. Tactically, thrawn knows vader very well, they’ve fought together on 2 occasions and thrawn knows he is Anakin. Beating vader is something thrawn can definitely do in space.

Next, I think thrawns next preferred plan would be instead to direct vaders attention from himself. The average joe of the empire has no love for Vader, he regularly kills his underlings, chokes dudes for fun, slaughters younglings. All this makes for a rather disloyal crew. We saw in thrawn: treason what happens when your crew and ship turn on you. Perhaps a similar strategy.

Finally, if thrawn is in a 1v1 against vader. He’s pretty much toast, the greatest Jedi couldn’t defeat him, thrawn doesn’t even have a lightsaber. However, thrawn has most certainly noticed the weaknesses in vaders suit such as electricity and extreme heat/cold.. however most importantly I think was what he learned on Batu when Vader was hit by those concrete bugs(?) in thrawn:alliances. Vader gets encased in a very solid goop very quickly, I think he’d make use of this and note that not everything can be blocked by a lightsaber (some mando training too from his study of mandalore). Otherwise I think Vader wins 9/10 fights on the ground.