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can i use psychic powers while embarked in a vehicle

Simple answer, yes. With several restrictions and/or caveats.
For example: Shooting power? Must have a firing point.

Was there a particular power you were wondering about?

fortune inside a wave serpent

I think you might be able to. It says in the BGB that you need a fire point (which a WS doent have) but i think that this may only refer to shooting powers.

You need a fire point for a shooting attack (or psychic shooting attack). Fortune, Guide and Doom are not treated as shooting attacks. Mind War and Eldritch storm are and cannot be cast from inside a vehicle (NLOS or not).

Farseer and unit embarked in a transport: Yes, he can cast it on himself or them.

Farseer casting on another embarked unit: No.
Farseer casting at a non-embarked unit within range of vehicle hull: Yes.