Finally acquired Virgil's serum. The Miscellaneous quest to give him the serum was still highlighted. Fast traveled to his cave; quest still active. Entered his cave, at which point I get a "Failed" message and everyone and everything in the cave attacks me.

Did I merely take too long getting back to him? Do I have the "wrong" Companions with me, making Virgil hostile towards us? Is it a bug? Is it a glitch? How can I be a nice guy and provide the nice scientist what he needs to NOT lose his mind?

You were wearing power armor with a targetting HUD in the helmet? Take off the helmet and try again.

Also, did you remove any of his traps like the cans or the turrets? If you do, he's hostile.

Of course, if you take too long to give him the cure, he's automatically hostile.