Only usable before range is chosen if you have a weapon card in your hand.
Equip this vampire with that weapon (and pay cost to equip as normal).
As above, but usable when choosing a strike.
Clarifications and Rulings
If Disguised Weapon is used to equip and contest a unique weapon possessed by the opposing minion, neither weapon is available for use during the combat. If the opposing minion had already chosen that weapon as his strike, then the strike fizzles. [LSJ 19980319]