I did expect something like that. To ply 150 grams of the blue into a sweater portion of yarn without loosing its quality is something I couldn't do. Can't you use this for an accent colour and make the main vest out of the grey Gotland? I'm afraid that if you blend it even more by knitting it every other row, it will fade into the background.
Ah, yes, it was wildly optimistic. But you were sweet enough not to burst my bubble : )
It might fade too much, yes - I shall give it a try, and see. I will have to spin up enough of the Gotland anyways, so I can do a tiny bit of test-knitting with it. If it fades too much, I will have to reconsider, and yes, accent colour would be a good option then...