The SR mixing system might hide which vendor you ordered from (although it might not if you load a unique amount of money that is associated with a certain vendors product, or other similar things) but it doesn't hide the fact that you use SR. And it doesn't use blind signatures so it offers no protection from anyone who has pwnt SR or SR himself. I wouldn't put any faith in the SR mixing system keeping you anonymous, but it is better than nothing. Look into Open Transactions.