Do units tranported in a vehicle have to shoot at what the vehicle is shooting?
No. Read your rulebook (
pg 64).
Does it matter if the vehicle is part of their units (deciated transport) or if its a transport anyone can use?
No, and a dedicated transport isn't part of the squad anyway.
Also I am trying to find a decent interpretation of the chaos Psychic power Gift of Chaos, in the power description it says the power may be used at the beginning of your turn. Does this mean that the beginning of your turn is the only time you can use it?
Yes. No "decent interpretation" necessary. The rule means what it says.