Overreacting to a somewhat overwhelming situation for the person twilighting, but for others, the situation isn’t that bad. The person tends to overthink, overplan, and hyperventilate.
Friend: Uh, hey, are you doing alright?
Twilighter: Hehe, yeah! I’m doing alright! Just need 7,432 things to do before the big event happens- OH WHO AM I KIDDING I AM NOT OKAY THERE IS SO MUCH TO DO BY TOMORROW!
Friend: Wow, you are twilighting hard.
It's a song, you sex-obsessed idiots! A perfectly innocent Owl City song about a guy who misses his girlfriend (or perhaps even a platonic friend, it isn't clear). Stop trying to turn it into something dirty.
When he uses the term, "Vanilla Twilight", he is literally talking about the color of the sky:
I'll find repose in new ways
Though I haven't slept in two days
'Cause cold nostalgia chills me to the bone
But drenched in vanilla twilight
I'll sit on the front porch all night
Waist deep in thought because when
I think of you I don't feel so alone
Twilightlicious is a brony meme originating from a vocal tweet by Tara Strong, the voice actor for Twilight Sparkle (A character in the rather popular My Little Pony: Friendship is Magic universe)
The format is a small rhyme, usually centered around the basic elements of the character (Twilight being the exception, as the fandom typically portrays her as more of a troll)
"I'm the T to the W-I, L-I-G-H-T, and ain't no other pony troll it down like me: I'm Twilightlicious"
Twilight Sparkle