Final answer:
The trough serum drug concentration of vancomycin should be collected immediately before the next dose is administered.
Step-by-step explanation:
The trough serum drug concentration is the lowest level of the drug in a patient's blood stream, usually measured just before the next dose is administered. For the antibiotic vancomycin, it is typically recommended to collect the specimen immediately before the next scheduled dose. This ensures that the concentration of the drug is at its lowest point, allowing for accurate measurement of the trough level.