Maybe that's the difference between a volunteer vaccinator and a nurse? Both mine were done by the same male nurse. Did it perfectly, exactly where he's supposed to - front of deltoid. Without a doubt super skinny arms pose a greater risk of hitting the major deltoid vessels if you go in the wrong place but we're all unique and many will have variations in physiology and there's smaller anterior vessels that are easy to reach on anybody. It's largely pot luck but you'd have to be seriously unlucky for the syringe to not only hit a vessel but stop perfectly inside it to inject IV.
Regardless, the guidance today is not to aspirate and until that guidance changes then nobody dishing out vaccines will do it, not even upon request. So the choice is to refuse the vaccine or close you're eyes, say a little prayer and hope you don't hit the 1 in 50k blot clot jackpot.
To me it makes perfect sense to aspirate but i wouldn't refuse the vaccine because they currently don't and I wouldn't put a gun to the nurses head either. Until the guidance changes it is what it is.