this is from another post i made but on it i try and justify where i put luffy strength-wise, im not saying luffy is fit to be a vice admiral, i just think strength wise he fits in this category well, also you have to remember people also gain rank for their knowledge an so on, so perhaps someone like franky or usopp could be high up, mainly because of their skills, i get this impression from how useful vegapunk is, and the level of protection he is given. yeah to sum it up i think on strength alone luffy can justify his position as vice admiral
other reasons
-luffy stoppig crocodile would have given his a promotion and fame
-luffy stopping arlong, wapol, krieg, kuro, baroque works, and so on just add to his career status, at least thats how i see it
1.sengoku (because he leads the marines)
2.admirals (because they are praised by sengoku, as well as being the most commonly used force against strong pirates/threats)
3.garp/john giant (because it was garp that chased roger, also john giant seems to control the other V-admirals and is based at the holyland, who aslo seems to train the elite, and was there one in charge of gathering the warships/marines for the buster-call)
4.luffy/komir/smoker (smoker because he is a logia)
5.momonga/doberman/onigumo