Vulnerability and Competing for the Part-Score
At pairs, going one down not vulnerable (-50) beats letting the opponents make a part-score. Vulnerable, -100 against their -110 is close.
When both sides have a fit, the part-score battle is common. Should you bid on, or let the opponents play? Vulnerability is a big part of the answer.
The arithmetic
The opponents bid 2♠ and will make it: +110 to them. Your choices:
Let them play: -110.
Bid 3♦, not vulnerable, one down undoubled: -50. Better than -110.
Bid 3♦, vulnerable, one down undoubled: -100. Still slightly better than -110, but only just.
One down doubled: -100 not vulnerable, -200 vulnerable. Vulnerable, that is worse than letting them make 2♠.
So competing is attractive when not vulnerable and riskier when vulnerable, because vulnerable undertricks cost twice as much and doubles hurt more.
When to compete
You have a good fit, ideally nine cards, so you are unlikely to be doubled and go two down.
Your values are in your long suits rather than in their suit.
At pairs, small margins matter: -50 against -110 is a real gain.
When to defend
Your values are in the opponents' suit: you may beat their contract.
You only have an eight-card fit and are vulnerable.
The opponents are likely to double you.
Key takeaways
Not vulnerable, going one down beats letting them make a part-score.
Vulnerable, competing is closer: avoid being doubled.
Compete with fit and shape, defend with values in their suit.