Since reading about it on Transfer and discovering that you can get them in Germany, I have tried soda crystals. I only knew of them for de-staining a teapot before,which is what my mother used to do. I think she also used to boil hankies with them. But soda crystals really do get rid of even baked in tea stains on a teapot.
I followed the instructions on the packet which was to dissolve the crystals in very hot water and leave for 30 mins then soak the clothes overnight then wash. My first attempt was with very badly stained seat covers, and it worked wonders.
From what people have reported here on GN, soda crystals are brilliant for loads of things and even good for the washing machine. And cheaper than Vanish, and probably less detrimental to the environment, to boot.