Vulnerability and Preempting
Preempt more freely when not vulnerable and need a better suit when vulnerable, because going down costs twice as much.
A preempt takes bidding space from the opponents, and the price is the risk of being doubled. That price depends on vulnerability, so your preempts should too.
What a double costs
Undertricks doubled:
Not vulnerable: 1 down 100, 2 down 300, 3 down 500.
Vulnerable: 1 down 200, 2 down 500, 3 down 800.
The opponents' game is worth about 400 not vulnerable and 600 vulnerable.
The four colours
Favourable (you not vulnerable, them vulnerable): preempt most freely. Three down doubled (500) is still less than their vulnerable game (620).
Both not vulnerable: a normal preempt. Two down doubled (300) is less than their game (420).
Both vulnerable: be sound. Two down doubled (500) is less than their game (620), but only just.
Unfavourable (you vulnerable, them not): be careful. Two down doubled (500) costs more than their game (420).
A practical guide
A common rule of thumb is to expect to go no more than three down when not vulnerable and two down when vulnerable, if doubled. Vulnerable preempts should have a good suit, usually with two of the top three honours.
Example
♠ K J 9 8 7 4 ♥ 7 3 ♦ 9 5 2 ♣ 8 (4 HCP). Not vulnerable, open a weak 2♠. At unfavourable vulnerability this suit is too weak: pass.
Key takeaways
Doubled undertricks cost much more when vulnerable.
Be bold at favourable, sound when vulnerable.
A vulnerable preempt needs a good suit.