Another unrelated question: When youre in a large counter war, is there any reason to target the original spell instead of one of their counters?
There is no general rule and it will always be different.
If you have Drain, threatening to target a Force of Will may be more juicy than an ACall.
If you have Misdirection in hand, you may counter their counter to bait them into over-expanding away from the ACall (they may end up mistepping their own ACall in response to your Misdi!).
Same thing, if you have Misdi, maybe you choose to exclude their ACall from your Mindbreak Trap.
If they have Snapcaster, Yawg's Will or Treasure Cruise, maybe you prefer to exile all with Mindbreak Trap.
If they have Snapcaster, maybe you don't want to give them a low CMC counter in the graveyard midway through the fight
With Flusterstorm, you often split the storm copies because if they let the one spell be countered, you may not want all the copies to fizzle and be wasted...
Then once you start mixing conditional counters such as: Misstep, Spell Snare, Spell Pierce, REB, Swan Song with your various removals: Nature's Claim, Bolt, STP, Dack Fayden, Repeal, Rav Trap... the scenarios are always infinite and interesting.
Unless you're ok to go with a "it depends" answer, I don't think giving you a rule#1 in Vintage is very helpful.