[blml] Obviously this is the prime duty

[David Burn]

[SW]

Of course South must _bid_ as if 2H is natural and non-forcing; 

[TWM]

Why?  The announcement may have drawn South's attention to the vulnerability
but nevertheless it is now AI to him that he opened 1N while vul.  It is
impossible for the vulnerability itself ever to be UI.

If he had forgotten the system there would be UI but there is no question of
that in your intro.  

[DALB]

Good grief. I had not thought it possible for anyone to write greater
nonsense on this topic than Herman, but I now see that I was wrong. One
might as well say, for example, that if while a player is considering his
line of defence against four spades, his partner says "We have three
tricks", the player may now choose to cash a fourth because the number of
tricks his side has taken is always AI to him regardless of how he has come
by it.

It is well established, because it is what the Laws say, that even if a
player realises before hearing his partner's explanation (or any other kind
of UI) that he has made an error of any kind, he must continue in his error
(because he cannot demonstrate that he realised the error before the
explanation occurred). In the given case, it does not matter whether when
South opened 1NT he had forgotten the system or the vulnerability; his
attention has been drawn to his error by his partner's explanation, and he
must not attempt to compensate in any way.

[TWM] 

Give the hand as a bidding problem to as many people as you wish (as a
screen test, no info from partner but opener has realised the vul by the
time he must make his second call) and I think 2S will be a universal
choice.

[DALB]

Of course it would. But so what?

David Burn
London, England