I personally shy away from single vials, as they are typically relabeled grey market vials, and finding any testing or source can be difficult. A few single-vial vendors have tests, but they are just kits purchased from a vendor with the vendor's tests. The only single vial place I bought from when I wanted to try a single vial was Polaris Peptides. It is pricey but worth it for time and ease of mind. I gained that information by reading and listening to experienced members here.*
*Do not consider it medical advice. Always consult a licensed medical professional—or at least someone who didn’t learn pharmacology from glp1 forum