Do you think it would alright to put a sachet of test into an empty test bottle that was already used? Or am I just being too risky?
Very bad idea IMO.
I personally would rather play it safe and get a new one. Not worth the risk
You should filter the gear from the sachets before using it. That being said, you can use old vials but make sure the rubber stopper is in good condition. If the stopper is deteriorating it won't seal very well and you may also get chunks of rubber in the oil. As long as you used sterile needles you should have no problem with vial sterility.