10 Iron gives you 100 EVs, which is 25 stat points at level 100. so you'll get one extra stat point every 4 levels you raise it.

if you're wondering why there wasn't a big boost the very next level after you fed the vitamins, you should check its stats before and after using them. vitamins force recalculation of stats. even at level 5, feeding 10 iron will give at least +1 to a stat that isn't lowered by its nature.

i don't understand your second question, though. evolving is completely separate from EVs, the two have nothing to do with each other. when a pokemon evolves, it keeps the EVs it had before, so a blaziken with 252 EV in a stat will have that stat higher than a blaziken with 0 EVs in it.