There's a lot more humans than vampires, and vampires can't fight during the day. Humans may not like being used as cattle for monsters.
Or we could treat hemovorism as a necessary evil and start up blood banks. Dur.
That would work... except that in VtM vampires are (apparently) inhuman. Even the Camarilla vampires, which theoretically use "humanity" as a means of maintaining self control, see actual humans as cattle and are usually somewhere between human and monster. Maintaining the masquerade is far more important than preserving human lives, even "innocents".
Other vampire groups don't even pretend to have human morality. They have various code, but the common element is that they all have to feed, and would be killed if exposed.
I don't know how much of that is "monstrous nature" and how much is just being an immortal human forced to steal blood to survive. Most of the codes seem to focus on keeping the monster in check, but the methods are drastically different (including cooperation). I wish I knew more about the setting actually...
According to some of the loading screens during Vampire: The Masquerade: Bloodlines, humanity became aware of Kindred and nearly drove them to extinction in the Middle Ages(using pitchforks and, more importantly, torches). In VtM:B, humans still outnumber Kindred by about 60,000:1(I think that was the ratio), and have access to automatic weapons with incendiary ammunition - all vampires do take only 50% damage from normal bullets and automatically convert to blunt damage(easiest to defend against), but I would think IN bullets don't entirely have that penalty.
I dunno how cut and dry it is. I think incendiary ammo would absolutely devastate most vampires, yeah. But the older ones get some insane powers. Particularly the antediluvians, the sleeping ancients so dangerous that the oldest active vampires are terrified of them. "Mind controlling entire cities" level powers.
I kinda wonder if the main reason for secrecy is that humans might awaken the antediluvians. A reveal would at least disrupt the balance of power, in a way that I bet centuries-old immortals would prefer to avoid.