RMS is a rate-dependent infusion reaction to vancomycin. The anaphylactoid reaction is from direct mast cell degranulation by vancomycin and subsequent histamine release. This results in pruritis, erythema, and flushing over the head/neck and upper torso. Anaphylaxis occurs via IgE and is less common than RMS with vancomycin.
So give antihistamines (diphenhydramine, ranitidine) and stop the infusion. Treat hypotension with fluids. After symptoms resolve, restart vancomycin at a slower rate (varies based on severity of symptoms). Consider true anaphylaxis if symptoms do not resolve and give epi and steroids.
Sources
Rosh Review
LIFTL
Uptodate