Each vitamins give 10 EVs for each stat respectively. So that means you have to divide the required EVs by 10 to find the amount of required vitamins, and round down. The rest of the EVs not divisible by 10 are to be obtained through other ways, eg. wild Pokemon, etc.
HP: 96/10 = 9.6 or 9 HP Up + 6 EVs
Atk: 252/10 = 25.2 or 25 Proteins + 2 EVs
Def: As 4 < 10, the Defense EVs can't be exactly obtained by using Iron. In other words, 0 Iron + 4 EVs
SpD: Same case as above. 0 Zinc + 4 EVs
Spe: 152/10 = 15.2 or 15 Carbos + 2 EVs
So, finally you need 9 HP Ups, 25 Proteins, 15 Carbos and the rest of 6 HP EVs, 2 Atk EVs, 4 Def and SpD EVs , and finally 2 Spe EVs from defeating other Pokemon instead. Hope this helps!