I Fear that players put too much emphasis on vulnerability. Some players think “not vulnerable” means invulnerable: They take liberties with preempts and competitive actions. If the same players are vulnerable, they may disappear from the auction like a turtle into his shell. But being vulnerable doesn’t mean you can take fewer tricks. Bid your hand.
In a team match, West’s double suggested support for the unbid suits, hence shortness in hearts. From East’s vantage point, if West’s values were minimum, North-South could make four hearts. If West had extra strength, East-West could win a bundle of tricks at diamonds. But East was vulnerable; he sold out to four hearts.
When West led the king of clubs, East ruffed dummy’s ace and erred by leading a diamond. South ruffed, drew trumps and next led his five of clubs.
If West played low, dummy would win, and South would pitch his jack of clubs on the ace of diamonds, losing two spades and one trump. If instead West took the queen of clubs, South could cash the jack later and discard three spades on the 10-9 of clubs and ace of diamonds.
At the other table, East refused to let the vulnerability turtle him. Over North’s redouble, he showed his hand by jumping to four diamonds.
After two passes, North tried four hearts, but West competed to five diamonds.
North doubled; he had two aces opposite South’s opening bid. But East had no trouble winning 11 tricks, adding plus 750 to the plus 420 his teammates had scored.