I have many viscose (including viscose Seasalt) dresses and I always wash at 40 degrees. The important thing is to put the wet item carefully on a hanger and dry it on the line or somewhere warmish in Winter. The viscose will feel 'hard' when wet and may even appear shrunk, but will soften back to normal as it dries. I never have to iron it unless the hem needs smoothing. I have NEVER known a viscose dress to actually shrink. Viscose crepes, which this dress looks like, can be ironed to make the crepe smoother, but personally I prefer the hang and drape when it is crepey.
There is so much tosh written here about viscose including that it is a man-made fibre. It isn't, it is made of plant pulps & fibres - as someone pointed out, viscoses include Rayon, modal, lyocell & bamboo.