question about Leblanc

Firstly you are correct that you cannot respond to any effect that is triggered as an EX Burst, so in that instance you can do nothing and Leblanc would be put into the Break Zone straight away.

In the situation where the opponent casts Mateus, you are able to respond, however due to the wording on Mateus, Leblanc's ability cannot save her. Mateus makes you choose your attacking Forward and put it into the Break Zone, it does not Break the Forward (this is a subtle but relevant difference). Leblanc's ability only prevents from being broken this turn, not from being put into the Break Zone, so it does not save her. Similarly a card like Titan would not save her from Mateus' effect for the same reasons.
Also note that another way that cards can be put into the Break Zone instead of being Broken is when their power is reduced to zero or less. So if the opponent was to use something like Black Mage (give a Forward -4000 power) on one of your Forwards with only 4000 power to start with, then they would be put into the Break Zone and could not be saved by Titan or similar effects.

For the Vanille question, all cards that refer to the cost of a card like this only work on the printed cost on the card, so Gabranth could never be put into play with Vanille. The reason for this is that the cost reduction mechanics like Gabranth have (or ones from Opus 1 like Yuna making Water Summons cost 1 less) only affect the cost to cast the cards from your hand. They are not static effect that permanently change the CP cost of the card. This is why Odin (break a Forward of cost 4 or less) can never break Gabranth, no matter how much damage you have taken. Odin still sees 7CP on the card and can't target it.
Note also that Summoner (Cast 1 Summon or cost 2 or less from your hand without paying the cost) cannot cast Leviathan even if you have Yuna in play. This is for the same reasons as above.

Sorry that was a long reply but wanted to cover it thoroughly. Hope it all makes sense but if not, please let me know

Yeah Mateus is very powerful. And i'm sorry to say that not even Carbuncle stops it, as Carbuncle only cancels Summons that target your characters, but Mateus doesn't target a character, it just says 'opponent'. The only thing that works against it is Summoner (1-053) which can cancel the effect.

I guess the other ways around it would be to return your Forward to your hand in response. You don't get the attack in that way but you can replay the Forward later if it is really important.

If everything is so literal I think I should be able to use Vanille with Gabranth it doesn't say 4 or less printed cost and his ability if you want to be literal only refers to playing him on the field once he enters play his reduction no longer applies