Hi, I'll give what suggestions I can with the info at least to offer some starting points for those two formulas. It would be helpful to know what the lesions looked like and what clinical signs in this patient are.
Generally speaking, Qing Ying Tang would be indicated for more severe signs and Si Wu Tang for milder presentations.
Qing Ying Tang cools blood and nourishes Yin. Patients for which it is indicated usually have significant Heat signs. Many have cool limbs and ears with a warm torso, red to purple-red or purple tongue, wiry pulse - may be choppy or irregular (if stasis is present). Skin lesions are usually severe and there might be a history of onset of signs shortly after vaccination.
Si Wu Tang is a Blood tonic. Blood deficiency skin lesions are typically mild - with thin powdery dander, mild itch, dry hair coat, improvement with fatty acid supplementation. Tongue is usually pale, pulse can be thin and weak or thin and wiry (maybe choppy or irregular if blood stasis is present).
If your lesions are severe, Qing Ying Tang is going to be more effective at clearing the pathogen and cooling the blood.
Hope that gives you a place to start.
Erin