In the hospital, if we were doing chemo drugs from vials, the vial top was considered sterile if you used it immediately after removing the cap. A second usage would then require cleaning with an alcohol wipe.
Note on worrying you are screwing up: Although it is good to be cautious, and clean the cap carefully, if you forget, there is very little risk to your health. I worked with many diabetics over my retirement career in public health, and I asked questions about this exact topic every admission to service. Many people reuse needles for testing, and also reuse diabetic syringes more than once. Same goes with air bubbles, it would take a huge amount to kill someone.Actually a small bubble at the end of an IM injection prevents fluid leaking up the needle track. Never had any issues with these things, that break every rule RN's learn in school. We won't discuss how many people who self-cath (for urine) rewashed the (one time usage) catheters in the dishwasher, some of you may be preparing to eat.