You can find the entrances and exits to the vents at spawn, on opposite sides of the middle of the map, and as an exit only, in the Intel room.

Once you've entered the vent at spawn, you can either go right to go to the Intel room, or go left to get to mid.

After entering from mid and going forward, you can either move up to spawn, or flank left to the Intel room.

If somehow entering from Intel (via Soldier's rocket jumps, Demoman's sticky/grenade jumps, etc.), you can go left to get to spawn, or go right to get to mid.

Both teams can use the others' flank paths via the vents. If you suspect someone's doing this, keep a close eye out, and get ready to fire once they peek the corner. Noisy weapons, such as the Miniguns (minus the Tomislav), are less effective for ambushing the stray enemies. Sticky traps are quite obviously the best if you can see when they're coming. Charging up the Huntsman is also viable, as the thing can instantly kill enemies if shot at their head, no matter their health. If one's sneaky enough, they could place a sentry in the enemy's vents to block them off until an Übercharge arrives. You didn't hear that from me, though.

Here's a quick map you can use to guide yourself through Turbine, taken from the TF2 Wiki.