Not A Bug Force of Vigor -

[CaptainThor]

I cast a Force of Vigor for its mana cost, and when it resolved it didn't give me a option to target an artifact I wanted to target. I wanted to target a Sword of Feast and Famine after it was attached to a creature but I didn't get the option to target it. I would like to if the sword is legal target in that situation.

 

[MTGO_TonyM]

You choose targets only when you cast the spell. It is legal to choose zero targets on any ability that asks for "up to #" or "any number of" targets, and this indeed happens if you choose OK when the prompt asks you to select targets (usually just after you click Force of Vigor).

So as a reminder, click all of your targets before clicking OK on these types of spells.