I believe this is Mitchie M's "FREELY TOMORROW", with Miku as the vocal.
From what I was able to garner, "Requiem" was included in its 1st DLC song pack, which means you would have to purchase it.
The first song that came to my mind is "clock lock works", which doesn't really fit the description really well. Hm, perhaps you might give it a try?
No no, I meant that he is more prominently known as SING's composer rather than a Vocaloid producer. Most of what you can find about him is connected to the girl group or other artists, despite the fact that he also releases Vocaloid versions of the songs.
Well, scratch that, I'm curious to see his activities too.
With regards to your inquiries, you may want to message NebulousViper to discuss, as he is in charged of the Chinese songs on the wiki.
Regarding T2o (李懋扬), there's relatively little information about him related to the Vocaloid field; more of a composer for the SING girl group. Might want to look through his Weibo for some info. You should also discuss with Viper about this.
Recently, there is a change in the url format of bilibili. There are now two formats for bilibili links, which can be used interchangeably: av(string of number ID) / BV(string of ID),
For example, BV1V7411G7Fj and av88782607 both redirect you to the same video【KAITO】花色【香椎モイミ】
I believe you are referring to a fanmade PV instead of the original PV of for this song (sm2528674). The original one only has a still illustration of Len throughout the whole song.
With regards to the fanmade PV, it's perhaps up to the PV maker's own interpretation, and not necessarily in correlation with what the original lyrics were intended.
This is "Palmtop Wonderland" by nekobolo ft. Miku.
Original Niconico upload: link
It's gonna be a lot. Start from "older" songs is essential, but it is also worth branching to newer ones. I recommend memorizing producers' name as well, if you like their specific style.
Some veteran producers I would recommend for starters:
Ryo with songs like 'Melt', 'Black Rock Shooter', 'When the first love ends', 'World is Mine', 'ODDS&ENDS'
Wowaka: 'Rolling Girl', 'Unhappy Refrain', 'World's End Dancehall', 'Two-faced Lovers', 'Unknown Mother Goose'
Tohma: 'Orange', 'Babylon, 'Envy Catwalk', 'Kowloon Retro', 'Skeleton Orchestra and Lilia', 'The Hanged Girl in the Haunted House'
Kikuo: 'Hole-dwelling', 'A Happy Death', 'Ten Sho Sho Ten Sho'
Kurousa-P: 'Senbonzakura', 'Risky Game', 'Cantarella' (original & grace edition are both masterpieces), Jougen no Tsuki
DECO*27: 'Ghost Rule', 'Streaming Heart', 'Mozaik Role', 'Coward Montblanc', 'Delusional Tax'
MARETU: 'Coin Locker Baby', 'Brain Revolution Girl', 'Mind Brand'
Kairiki Bear: 'Venom', 'Angel'
Neru: 'Tokyo Teddy Bear', 'FPS', 'Jailbreak', 'Dappo Rock', 'How to Sekai-Seifuku'
Kemu: 'Life Reset Button', 'Six Trillion Years and Overnight Story', 'Reincarnation',... Basically the whole KEMU VOXX series
Toa: 'Tsugihagi Staccato', 'Music Music'
Orangestar: 'DAYBREAK FRONTLINE', 'Night Sky Patrol of Tomorrow', 'Spinning Sky Rabit'
Sasakure.UK: 'Negiposi*Continues', 'Spider Thread Monopoly', '39'
Mitchie M: 'Ohedo Julia Night', 'Ai Dee', 'Viva Happy', 'Freely Tomorrow'
MikitoP: 'Roki', 'Sarishinohara', 'Akaito'
DATEKEN: 'Tsumugi Uta', 'SIGH', Step by step'
WONDERFUL★OPPORTUNITY!: 'Rimokon', '8HIT'
Rerulili: 'Brain Fluid Explosion Girl', 'Sacred Spear Explosion Boy' 'Donor song', 'Kami no Manimani', 'Knife'
Niki: 'WAVE', 'Grave', 'GLIDE', 'HYBRID'
EZFG: 'Cyber Thunder Cider', 'Hurting for a very hurtful pain', 'magician's operation'
PinocchioP: 'Slow Motion', 'Common World Domination' 'Tonchinkan no En'
Jin: 'Kagerou Daze', 'Outer Science', 'Children Record'
N-buna: 'Sea Lily Deep Sea Tale', 'Toumei Elegy'
40mP: 'Love Trial', 'Karakuri Pierrot', 'Torinoko City'
Giga-P: 'Childish War', 'GimmexGimme', 'Plus Boy'
The more contemporary producers include:
Hachiya Nanashi: 'Limelight', 'Ope', 'Neurosis', 'Shabon'
Harumaki Gohan: 'Dreamless Dreams', 'Aster', 'Melty Land Nightmare', 'Reunion'
Ayase: 'Ghost City Tokyo', 'Happy Ender'
R Sound Design: 'Teikoku Shoujo', 'Dante'
DIVELA: 'Vocalo-Colliseum', 'METEOR', 'Dark Rom Syncer'
SLAVE V-V-R: 'Hit You 8759632145 Times With This Piano', 'You're 56876423 Years Too Early to Find Food', '?'
*Luna: 'Manic', 'Chatlüge', 'Magic of Spring', 'Magic of Summer', '8.32'
Mafumafu (utaite): 'Jigsaw Puzzle', 'Merry Bad End' (he's actually produced songs long ago, but until recently that his songs become more popular as he switches to a more specific style)
NayutalieN: 'Alien Alien', 'Taiyoukei Disco', 'Dance Robot Dance'
Ewe: 'krank', 'Dancer in the Dark', 'quiet room'
Balloon: 'Mabel', 'Charles'
Eve: 'Dramaturgy', 'Last Dance', 'Outsider'
Nyanyannya: 'Doctor=Funk Beat', 'El Tango Egoista'
Hanyuu Maigo: 'Harehare ya', 'Zange Mairi', 'Aun no Beats'
There should be a lot more, and that's just Japanese alone I mentioned. Let YouTube and famous Vocaloid/Utau tuners lead your way on finding good songs.
World is Mine and probably Ievan Polkka. A bit deeper into the fandom, it would arguably be Rolling Girl, Senbonzakura or ECHO.
I do feel the same haha, you're not alone at all. It's a bit irritating seeing comments like "I love (song name) of (Vocaloid name)", etc. Kinda like how we only credit the performer's name, but not the people on the production crew whenever we mention a song.
But then again, I cannot force my thought on everyone, it's not like all care about producers like we do. On the other hand, I feel that the "no credit" mindset has subsided over the years.
That's how, whenever I mention a song, I would casually put the main producer's name next to the Vocaloid. Nothing wrong doing this haha.
Anyway, my advice for you is to try to utilize YouTube recommended list. Listen to not only old, but new songs as well.
When you find a new song that you like, try to listen to several more songs of the same producer, as there's a chance it would have a similar style. How do you know the producer? Look into the description, or search for the name on this wiki, Vocaloid Lyrics Wikia, or VocaDB.
I respect covers makers a lot, and there are many users you definitely shouldn't miss out. Do check them out if you can.
KAITO was the first Vocaloid I knew about, and he's been my favorite for almost 8 years. My favorite songs don't necessarily feature him however, I just enjoy good music in general. I would list some big name producers and my personal favorites, and you can start branching from the list.
For KAITO, I love the two Cantarella versions along with Jougen no Tsuki, all of which are Kurousa-P's songs. Senbonzakura is his most famous song.
If you are into metal/rock or some sort, Utsu-P is best-known for this genre. I'm not a fan of his, but maybe you would be interested?
Niki and EZFG were my favorite for electro music. But if I were to name a current time producer, Yunosuke is worth the mention. Then there's Giga-P who is super big too (with songs like Gigantic O.T.N, Hibikase or Rettou Joutou), I don't like his style but it's addictive in a sense.
You would probably have come across Wowaka, who is known for songs like Rolling Girl, I suppose? He's a special and talented producer who left us too soon, I hope you can listen to his songs too.
Other personal favorite producers I would recommend are Kanzaki Iori, Balloon, ewe, Hanyuu Maigo, Harumaki Gohan, Tohma, Kairiki Bear, kaoling, Hitoshizuku-P, toa, etc. A lot that can't be listed haha :') But you should go look for a favorite producer yourself.
I really can't name a specific song I like, there are so many haha.
Is いたいあたしあしたあいたい (Itai Atashi Ashita Aitai) the one you are looking for?
DECO*27's 'Once Upon a Me' partially fits your description, I wonder if it's what you are finding?
It has an article on our wiki: むかしむかしのきょうのぼく (Mukashi Mukashi no Kyou no Boku)