UNEXPECTED EXHAUSTION

If Manticore Venom is already present on the target unit and that unit is already vulnerable, does that prevent Unexpected Exhaustion from being attached to the unit?

No.

Follow Up: Since the token from the card was never placed, does that mean that even if the existing Vulnerable token on the unit is removed, since it was not the token from Unexpected Exhaustion, the card is not removed?

No, when you remove any Vulnerable Token on the unit while Unexpected Exhaustion is attached, the tactics card is removed at the same time, even if that token originally was present from another source.

If a unit cannot gain Vulnerable Tokens, (for example, Night's Watch War Machines) can the card be attached to them?

The card would immediately fall off the unit.

UNEXPECTED EXHAUSTION

If Manticore Venom is already present on the target unit and that unit is already vulnerable, does that prevent Unexpected Exhaustion from being attached to the unit?​

Follow Up: Since the token from the card was never placed, does that mean that even if the existing Vulnerable token on the unit is removed, since it was not the token from Unexpected Exhaustion, the card is not removed?​

If a unit cannot gain Vulnerable Tokens, (for example, Night's Watch War Machines) can the card be attached to them?​

If Manticore Venom is already present on the target unit and that unit is already vulnerable, does that prevent Unexpected Exhaustion from being attached to the unit?

Follow Up: Since the token from the card was never placed, does that mean that even if the existing Vulnerable token on the unit is removed, since it was not the token from Unexpected Exhaustion, the card is not removed?

If a unit cannot gain Vulnerable Tokens, (for example, Night's Watch War Machines) can the card be attached to them?