A leather jacket will not protect anyone from Firearms damage. For one thing, armor isn't rated based on its "bashing"/"lethal" protection. Instead, it's rated for "general" vs. "firearms" protection:
Armor provides two kinds of protection: against general attacks and against Firearms attacks. [p. 170]
(Indeed, "general" attacks may include bashing, lethal or even aggravated damage.) All non-bulletproof armors are rated 0 against Firearms attacks. Even if leather armor was rated at "1 bashing" protection, I think a GM would still be obliged to infer that bullets pierce such armors according to the Piercing Durability sidebar on WoD p. 138.. but then, that's what the actual rating and rules are meant to represent.
Iron Skin is tricky, but I don't believe it applies to vampires and Firearms. Part of the problem is that it uses unique terminology:
[Your character] has an effective armor trait of 1 against bashing attacks only. [p. 111]
It's never made clear exactly what a "bashing attack" is, but the only other mention of it I can find does make it clear that it's not synonymous with "attacks that do bashing damage":
Certain targeted bashing attacks may cause lethal damage .. [p. 174]
So, a kevlar vest (which downgrades Firearms damage from lethal to bashing) doesn't mean Firearms attacks against the user are "bashing attacks". On this reading, it seems pretty clear that Firearms attacks are "lethal attacks" which can ultimately cause bashing damage in some cases (kevlar, vampires, etc.).
Finally, if you read the text of the merit, it says the character has "hardened his body to physical blows," and no amount of kung fu is going to harden one's body against bullets. Considering bullets affect vampires and humans equally in physical terms (it's just that vampires don't care if their organs have holes in them - VtR p. 170), I don't see any reason why kung fu would help a vampire in this context and not a human.