Use plain sodium thiosulfate, regular fixer agent is ammonium thiosulfate and is very harsh to the delicate silver image of Vandyke. Again: 2 minutes in 2% is about right. There will be very little unreacted silver in the paper which is very open/available to fixer's action owing to the fact that the paper doesn't have a superhardened gelatin layer - unlike S/G. (In other words; you don't need much thiosulfate to properly fix Vandykes or any other iron-silver processes...) I have prints processed that way which are on display since end of 2003 w/o a slightest sign of deterioration. (Both toned and untoned - but you know my preference...)