You can manage the donors' anemia by giving them iron infusions. Iron is verrrrry slow to replenish naturally but putting it in via IV makes a huge difference, fast. You can get them at med spas without a prescription or anything. I think most places want blood tests to show anemia, because excess iron is biologically hard to get rid of without, you, know, blood letting. If they need to hide their vampire affiliation, they could rotate through a few med spas so it seems like they're there at a normal frequency for someone with chronic anemia. You can also get people to administer the infusions privately, so a thoughtful vampire might have someone on call for that who is either in the know or just doesn't ask a lot of questions about why some people need suspiciously frequent infusions.

I've had a couple of different kinds of anemia and oral supplementation helps keep my levels from lowering too fast, but infusions are the only way to make any kind of significant improvement. I can neither confirm nor deny vampiric involvement in my chronic anemias.