You can sort of classify the events by how they are similar:
110h, 100, 400, LJ, HJ, PV - fast twitch
shot, jav, disc, PV - upper body strength
400, 1500 - speed endurance
Clearly, the 1500m is NOT the best background for the decathlon, since it doesn't translate well to the other events. It only really helps with the 400m. If you specialize in the PV and 400m, you have most things covered. Here are some possible decathlon backgrounds and their strengths:
mid-distance: 400, 1500, (HJ)
sprinting: 100, 110h, 400, LJ, HJ
throwing: shot, jav, disc
pole vault: 100, 110h, (400), LJ, HJ, PV
So yes, Pole vault is probably the best background for a decathlete, followed by pure sprinting. Mid-distance is probably the worst, because at least throwers have their events nailed down. The 400 and HJ, while still good events for the mid-distance runner, will still be won by sprinters or pole vaulters.