Hmmm, interesting question. Why are you using a presealed vial to mix HCG? Usually people mix about 1-3 cc's of BA water with HCG, so the result will fit in a single 3cc syringe. You can just store the syringe in the refrigerator and reuse it for multiple shots (clean the needle with alcohol each time). Syringes are a lot cheaper than presealed vials.
I don't know if HCG "spoils" but somehow I wouldn't want to trust something left in water at room temperature for several weeks. If you refrigerated the empty vial, it might be ok. Also you could try adding a little BA alcohol the next time you uise it to mix HCG, or you could sterilize it in boiling water (vent the top with a pin).