How quickly / easily can you acquire a vintage heat sink?

I got one on the station from a container I think.. you can save-scum at a loot container until you get one.. but honestly, it is a horrible mod - especially on insanity. An avenger seems to fire maybe 10-12 rounds before overheating for far too long.. it drastically reduces your ability to kill things in a reasonable time frame... I would only ever use it as a last-ditch weapon if all my other weapons were empty... but honestly I find equipping 3 weapons (3 pistols or smgs especially) to be be pretty great.. excellent ammo supply.. just keep swapping to the next pistol / smg when your current one runs out.

The heat sink seems to exist for nostalgia rather than functionality.

Really early. I got one just after the first mission on eos or maybe it was during that mission? Either way it wasn't long in. I didn't make mine I found it. So just be sure to explore early on, you'll find it.

I knew they'd do something like this.
They love their fucking thermal clips too much to potentially insult it.

It can be bought for research points on the Tempest without prerequisites. I had enough research point by the middle of the second planet, so like 10h into the game.

I can see that being useful for sniper rifles too.  They don't have much of a clip size either, and don't carry a whole lot of rounds normally

Get some Remnant guns instead? I have mostly only read the descriptions, but they all seem to work with the cooldown mechanic instead of requiring ammunition heat sinks. I do have the Equalizer SMG in MP. Very effective short to medium range weapon, cools down really quickly, even if you overheat it.

Adding Clip Size mods does not work. Adding this to the Gahn Shotgun is useful for Vanguards who keep moving and dodging in and out of combat because you cannot reload while dodging. 

Yip.

I put the VHS on the Pathfinder Pioneer AR with the extended mag and I actually enjoy the results, but my next weapon with it will definitely be a sniper rifle. I'm so glad we get the augs back on dismantle though. That's a big relief.