Car: 1986 300E (130k/mi)

I have noticed recent rust stains in my new cooling tank resevoir. My question is -- how serious is this, and can I do somthing to prevent further corrosion -- if this is indeed the problem. First, some background. . .

Last month I had the head-gasket replaced becuase it was leeching oil into the coolant. I also replaced the radiator, and all hoses. For better or worse, I flushed the remaing oil in the coolnat system by driving it (short distances) w/ a mix of water and vinagar. I used such a mix, changing it daily - for about four days.

Have I caused a corrosion problem, can it be reversed, --- or did I just loosen some remaining rust. I have since-of course, put a proper mix of coolant and water.
(PS: The stains may even have occured before I put this proper mix in --and they are not worsening.)

Any insight?

Steve