i'm sure cian heard a lot about vash and his disastrously short time at the academy but probably dropped off from caring or paying any mind after he left and it stopped being a passing topic at the dinner table. so maybe that on top of short passing exchanges during their smuggling days could bring about a slow realization when he's patching him up that "oh this is the same idiot" which would baffle him as much as it'd fuel him encouraging every wayward idea vash ever shares because while he thinks it's ridiculous he also wants to see the outcome and appreciates the job security in repeat injuries