Veils are blocks.
When a block is broken, it goes away.
For example: John is holding a door shut on Mike, Steve, and Tim. This is a form of block and if one of them break the block it's gone. John could establish the block again on his next action, but once Mike forces his way through the door then Steve and Tim can go through as well.
For example: Tim the Enchanter has an air shield up, blocking incoming attacks. Call it a block of 5 - Superb. If Mike tries to punch Tim and gets a Great result on his fist roll than the block is still then when John attacks. If John attacks and gets a Fantastic result then the block is down and Steve can attack without worrying about it.
So we have statement one:
The game treats a veil as a block on the senses. See YS252 and YS295 for proof of this.
Statement two:
Once someone overcomes a block it goes down. That's in the basic rules.
Which leads to statement three:
When someone breaks a normal, non-glamoured veil then that block is broken and the veil goes down.
Should the game treat veils as blocks? That's another debate, but in the rules as written they just another block and treated no differently than an air spell that blocks incoming attacks.
Richard