Since most vocaloid songs have lots of theories or meanings, these are just my personal understandings of the songs.

⚠️ And uh ! TW ! because some of these songs have sad / depressing / triggering topics and meanings. ⚠️

Aishite, Aishite, Aishite by Kikuo ft. Hatsune Miku:

A song about a girl, who finds a cursed necklace. This necklace makes her want to feel loved more and more, OR a hyperbole on golden child syndrome.

And you became the moon by Kikuo ft. Hatsune miku:

A song about two children falling into a lake, dying of hypothermia. At the very start of the music video, you can see two figures falling into something--the children falling into the lake. One line says "And then I crumbled and fell apart, and the moon fell into the open hole" which could mean that the lake cracked underneath them, the boy falling first, and looking at the hole he fell in, the girl staring down at him, the "moon". After the boy falls in, the girl also falls, both of them slowly dying. Another line says "But I can still see, inside the warm pocket I was placed into...the painful trembling is kindly, letting me sleep comfortably..." When you get hypothermia, you can suddenly feel warm, making you think you're cured or safe, but it just means you're closer to death. I would elaborate more, but I think you get the idea... This song is so sad... TwT

O Light by Kikuo ft. Hatsune Miku ( Also has a Hanatan cover ):

A song about a girl getting trapped in a coma after a car crash with a deer ( since there's many deers in mv, and she makes a triangle with her fingers with the deer, like a wild animal sign on the road; there's another reference to this at 2:03 in the music video ) Trapped in her coma, the girl can't escape her nighmares. At 2:39 in the music video, the girl is seen climbing a ladder, which could represent how difficult it was to move after the car crash, maybe her trying to crawl towards help. It could also represent how difficult it is to get out of her coma. At 2:58 in the music video, a figure is seen pressing a button, and the girl is freaking out, and becomes sad for the rest of the music video. I think this is a doctor taking her off of life support, because after a patient is in a coma for a while, the doctors give up and let her rest in peace. This, therefore, means that the girl freaked out because she would no longer have any chance of escaping this nighmarish place. The lyrics also say "Come save me, O Light, I'm waiting forever and ever, forever, forever..." which could mean that she's waiting for someone to save her, but since there's no chance of being alive again, she's just trapped, waiting an eternity for no one to show up. If that makes sense.

Hitowagari ( Subsitute ) by Kikuo ft. Hanatan ( Kikuohana ):

A song about a girl who was in a past abusive relationship. After leaving that relationship and being in a new, less toxic, relationship, she can't feel comfortable and happy, since she is only used to a more abusive relationship. The song lyrics say "I desperately need a substitute in my place" meaning that she doesn't feel worthy of this new relationship. The saddest part is that, in the music video, her center of gravity is her neck, meaning she's hanging from somewhere--it's clear that she hung herself, after not being able to be happy in her new relationship. At the very end, you can *slightly* see her feet, and they don't look like she's standing, it looks like they're being lifted. Hanging from somewhere. Sorry for the long explanation lol, this is one of my favorite songs.

( And uh, technically this isn't vocaloid, but I still wanted to include it srry! )

Tokyo Teddy Bear by Neru ft. Kagamine Rin:

A song about a boy who takes his own life, as shown in the first portion of the music video, basically saying "goodbye". Throughout the song, it basically explains why the boy commited su!cide; the song talks about him taking parts and cutting himself--this could basically mean how he changed himself for the expectations of others, like his parents, and would basically do anything, even change himself, to be accepted and loved by others. One of the lines says "The stitches are being undone and torn apart" which means that he eventually realizes that he's not himself anymore, and gave up on trying to meet the expectations of others. After this, he decides to take his own life.

That's all for now! Should I make a part two?