Today I was using the Phantom 2 Vision+ to record some land. I practiced GPS mode in an open area before i recorded, and I was happy with the outcome. Then I decided to record the track of land. I set every point for 150 feet high, but when it took off, it only went up about 10 feet and ran into trees. Luckily, I was able to get it down with little or no damage to the Phantom. Why didn't it go as high as I typed in?

Sounds like you are talking about using Waypoint mode in the DJI Vision app because you state "I set every point for 150 feet high".  You note that the Phantom only initially went up about 10 feet and then ran into some trees.  If you were using waypoint mode and you set the first waypoint some distance from your home point, this could explain what you observed.  The Phantom 2 V+ in Waypoint mode will rise to 10-15 feet above the Home Point and then fly directly to the first waypoint increasing its altitude along the way, so that, in your case, it would be at 150 feet altitude by the time it arrived at the first waypoint.  To avoid the problem you experienced (crashing into trees between the Home Point and the first Waypoint), you should always set the first waypoint at or very near the Home Point and set your starting altitude there.