First thing to check is exposure. Under-exposed Vandykes will bleach horribly when fixed (I always overprint slightly, knowing that they will bleach back a bit). Second thing is your fixing regimen. Your hypo is far too strong (I generally use 1 heaping teaspoon of hypo crystals to 1 liter of distilled water), and you're fixing time is far too long. 1 to 2 minutes should be plenty.