Although you would understand this simple interaction if you actually read cards, I have to write about this because people are retarded.
Geminis get through Effect Veiler and Fiendish Chain.
See, both Veiler and Fiendish Chain specify an effect monster as the effect target.
Hence, you cannot activate Fiendish Chain on things like Another Neos. It might pop up in a game or 2, eliminating their ability to stop your attack
And you cannot Veiler Gemini monsters before they re-summon.
However, you can veiler/chain one before it gets its effect. Heres how:
They have a gemini out, and you have a Fiendish Chain set.
The use their normal summon to change their Gemini to an effect monster. This does not start a chain but can be negated via solemn warning, etc.
This resolves, and priority is passed to you (the opponent) to respond to the summon.
At this timing, if you throw Veiler/Chain, you can stop the effect, as the chain is triggered by "link 0" of the effect monster being successfully summoned.
However, with supervis, as it re-summons via an effect, you cannot stop it with Veiler.
Lets assume you have a fiendish chain and a Sangan set.
Youre an idiot because you didnt set that torrential instead, but oh well. And a Reckless Charge or something.
They special summon Heiratic seal of the Dragon king via Swing of Memories.
At this point, it is still a normal monster, and hence cannot be Chained.
They activate supervis, targeting the seal.
Supervis goes onto chain 1, and whatever free chains happen after that.
However, as the seal is still a normal monster before Supervis gets attached, it is not a legal target for your fiendish chain
Once supervis(chain 1) has finished resolving, however, turn player has priority to activate their effects. Hence, the turn player retains priority to activate any effects(Seal of dragon king) before you can respond.
If they do something like activate something else, enter another phase, etc, you can respond to that by activating your Veiler/Chain
Edit: So apparently you cannot retain priority and activate spell speed 1 things. My bad. So you can be veiler'd. It will boil down to you explaining how gemini's are normal monsters while face up on the field, effect veiler can only hit effect monsters, and pray they dont remember it "Becomes an effect monster"