VIN raises heat, power and danger when it's higher. Though it's not as severe on impact as Vcore. It can go places more liberally.
It's the input voltage of the internal voltage regulators, something like the internal "PSUs". The Vcore actually drives the cores.
So, it has an impact and it needs being careful but the overclock itself is mainly affected by the Vcore.
In practice the VIN stabilizations are indirect since they result to stabler resulting voltages.
I'm not sure because those specs are confidential nowadays I think. It might be that Intel has a default of 1.8v across the whole range. But, motherboard manufacturers seem to do whatever they want lately and use whatever they want on VIN or Vcore regardless of VID or intel recommendations so it's hard to tell.
And it differs depending on bios version sometimes.