My understanding is that the Grayvik branded hammers are the "Seconds" or "blemished" hammers from Vaughan. Basically if they produce a batch of hammers that have dings or some other imperfection, Vaughan puts the blue paint on and labels it Grayvik. Some hammers of an older design they've stopped making and other items they're trying to close out may also get labelled Grayvik. One of the people from Epsteins posted some info on the Grayvik stuff in a past thread.
I THINK grayvik is like a factory second line of Vaughan. IIRC what I have read is if the item (say hammer for example) doesn't look as good as it should for the Vaughan name, but still works find, aka cosmetic flaw only. They slap the Grayvik name on it and sell it at a reduced price. A way of making a few bucks off a factory "oops".
I bought 2 grayvik tinners hammers and they are perfect. The head had paint as the vaughans are bare. I took a file to it. Great price. I couldnt be happier.