If there is a puddle of oil on the ground and a big haystack in front of me, and I hit them with a whip, I cannot in any way be said to have damaged either one despite my successful strikes.
If the whip is flaming, the oil will ignite, as would the hay, despite the fact that the whip-strike still did no damage to either target.
In fact, I daresay that I could ignite EITHER target while simply TOUCHING them with a flaming whip.
(This, of course, could give rise to a "new" tactic: if you know X creature is vulnerable to a particular kind of attack, you could eschew a regular attack and try for a touch attack in order to increase your chances of exposing X Creature to that power.)
Look at the mummy: DR 5/, but vulnerable to fire. A halfing with a +1 flaming dagger and 8STR fighting a mummy can't overcome the Mummy's DR with his weapon. And yet he can still hurt the sucker despite doing no damage because (tah-dah!) his weapon is a flaming weapon.