Tiling Question?

Ok, I can get the old vinyl tiles up pretty easily. What is underneath is concrete in good condition and some dried up adhesive from the old vinyl tiles that were put down. I can sc the dried up glue away and clean up with no problem, but then what I have is concrete with what I'm guessing is just some of the old adhesive that has maybe "soaked" (for lack of a better term) into the concrete, kind of discoloring it a little. Is it ok to tile over this? I really don't know any other way, it's not like you can remove it. Any ideas?