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Longstrike gives his vehicle overwatch. Moreover, it gives him the ability to overwatch multiple times per turn and supporting fire.
There is another Tau Point Defense Targetting Relay that gives vehicles the ability to overwatch, and supporting fire.

I don't have the 40k rulebook with me so I apologise if this is covered in the book, but:
Vehicles cannot be 'locked' in combat.
You can overwatch once per enemy assault phase as long as you are not locked in combat.

Does this mean that a vehicle that gets charged can decline to overwatch against the charging unit, but instead use its overwatch supporting fire to help out another Tau unit? And Longstrike's tank which can overwatch multiple times, it can overwatch at the unit assaulting it AND all other units assaulting other Tau within 6"?

Yes, that all seems right.

Yep, and on an ion head thats a lot of dakka to pile out....