I'd suggest acid all the way through at least until after fixing - I'd
avoid tap water until after fixing also.
If you use an alkaline fixer before _all_ traces of iron salts are
removed then your prints are I think very likely to fade. If you only
give a 1 minute wash (as KOL suggests) before fixing I'd expect
problems.
Fixing as normally thought of is to remove insoluble silver compounds
from the print. In VDB there should be little or no insoluble silver
compounds so they don't really need fixing like a salt print. Of course
when you coat paper with a silver solution some insolubles will normally
be formed, but most of the silver just washes out with the water rinse.
One reason to use several changes of distilled water for the initial
rinsing rather than doing it with tap water is to avoid any
precipitation of silver chloride etc - after a few rinses have removed
most of the soluble iron it is safe to add a clearing agent too. I think
I used about 5 mins in total with several changes rather than the 1
minute in KOL. (If tap water is alkaline it will also precipitate some
iron(III) in your paper.)
Keepers of Light recommends a 5% neutral sodium thiosulphate, but I
think this is stronger than needed and it would be better to be slightly
acidified. I think I used 2% with a pinch of citric acid, but don't have
my notes to hand.
I got little or no noticeable loss in highlight on fixing.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/