Buffy the Vampire Slayer is known for using unconventional and unexpected language for humor. This is not a "correct" phrase, and I would be cautious in repeating it.
The phrase Buffy is starting with is "to [blank] something up", meaning to enhance something with something else. For instance, "to dress that up" might mean to make something more fashionable. "To oil something up" means to apply oil.
Buffy is using the innappropriate word "vague" (meaning unclear and/or incomplete) in place of the expected verb in this sentence. In doing so, she is implying that Giles should treat "vague" as a verb here - in essence, to make something more vague.
So, the joke could be delivered as:
Giles: Something's coming, something, something, something is - is gonna happen here. Soon!
Buffy(Sarcastic): That was too much detail. Please be more vague.
However, because Buffy is a teenager who thinks technically incorrect English is funny, she instead mixes "vague" with another, technically inappropriate phrase. This lets her be sarcastic in an original and unexpected way.