(This is from someone who has only browsed Vocaloid fanfic once in 2013, browsed it more around the time I started getting back into it (2017-18) and has only written one very short work recently)
Hm, it's not as active as it would be, considering the hype started at 2010 and has fallen off through the years. However, there are new fanfics being posted and updated daily on both fanfiction.net and Archive of Our Own. Most of it focuses on Cryptonloids + Gakupo and Gumi, although there are a fair amount of other, less popular characters that are being featured, and ships are mainly the same, nothing changed on that front.
AO3 is a newer site, and has started to eclipse FFN in popularity. 2050 fanfics in AO3 is not bad compared to 20K fanfics in Fanfiction.net, especially considering some other fandoms. I haven't browsed Wattpad and Amino, but I feel that Archive has the most quality in fanworks, on average (could be biased though, but I don't have faith in FFN as a website, it has some issues, but I still).
Unfortunately (for me at least), the non-Japanese (more narrowly, the non-Crypton) Vocaloids aren't much focused on, which reflects a lot of people who have listened to Vocaloid as a whole, but on FFN, 5/6 Cryptonloids have at least 3000 fics about them while Oliver, the most popular Engloid on the site, has around 300 fics, while SeeU has around 100. Yanhe approximately 50, and Maika 30. Pretty depressing, but considering the Chinese and Spanishloids are more recent, eh...and on the other hand, Teto, Haku, and original characters have more than 100 fics. (EDIT: This is not meant as bashing, there is a lot of overlap between voice synths and I like what they offer) AO3 is much the same - Oliver has around 100 fanworks to his name, while Len, the character with the most works on that site (and on FFN) has about 700.
Oof, only around 1% of this post is about how active this is and the rest of it is a rant, sorry that this got kinda into other topics. But write what you want to write, don't take too much of what I said to heart when writing it, it is your work first and foremost.