Rats!

Are there any alternative distributors that allow Vocaloid and other Vocal Synths?
You can use any music distributor (DistroKid, LANDR, TuneCore, etc) to distribute vocal synth music, so long as you don't put the name of the voice bank in the metadata or use their likeness in album art.
Using the voice itself is fine...because obviously someone would be laughed out of town if they tried selling music tools with a license forbidding their use to distribute music lol. But the name and likeness are trademark territory and, for example, Miku merchandise is big business, so Crypton is incentivized to look at that on a case by case basis.
The two paths, in the Crypton example, to having a character on the album cover and metadata would be to contact them and ask (unlikely for people outside of Japan) or being signed by their Karent record label. That's why you might notice that 4BLOOD by KIRA has no mentions of Miku on streaming services, but Highlight (a song commissioned for Miku Expo) does. One's through KIRA's distributor and the other was published by Crypton.
Also, Crypton basically considers anything on YouTube, NicoNico, or other social media platforms to be "doujin activity" and their stance is that it benefits them so do what you want. Fan art away and say the song is featuring Luka or Miku or whatever. They know they have their golden goose because of people making videos with Miku in them.
Personally, I use LANDR. It's $24/year for unlimited releases (and they don't delist them if you stop paying, unlike DistroKid) and they don't take any portion of the royalties. They will also, for a per-song fee, arrange the paperwork for you for licenses for cover songs. (i.e. an original performance of a song...sampling existing recordings is a very different legal area.)