I've noticed a major chunk of Vitamin Connection posts that have been deleted for either being unapproved for more than 30 days or irrelevant to site(Human only). I am unsure as to what the latter means. But is there a reason why Vitamin Connection posts have a high chance of being deleted? Are there other franchises affected in the same way?

List of deleted Vitamin Connection posts:
https://e621.net/posts?tags=status%3Adeleted+vitamin_connection+

If you're curious why a post is deleted, it's best to send a polite DM to the moderator that deleted it (or in the case of the auto-mod, to the head admin, NotMeNotYou). You can click their name in the deletion reason to go to their profile, where there's a link to "Send message".

As for my opinion on why they could have been deleted, at least of the ones I saw that have a proper source (not empty sources, or only an artist site link that's not to the image in question), they do seem quite close to looking like simplistic toony humans in space suits, to the point that no one felt comfortable approving it and either got autodeleted after 30 days, or someone felt like it wasn't furry-related and expressly deleted it first.

fireicewater2 said:
I've noticed a major chunk of Vitamin Connection posts that have been deleted for either being unapproved for more than 30 days or irrelevant to site(Human only). I am unsure as to what the latter means. But is there a reason why Vitamin Connection posts have a high chance of being deleted?

Watsit's assessment is correct. They often look like toony humans wearing spacesuits from the 1940s or '50s, but it's still very borderline. If it gets to the 30-day deletion by the automod, there's often at least one staff who's disapproved of it on relevancy grounds, but weren't confident enough in their decision to outright delete it. If a picture of these characters has been approved, such as the one you're using for an avatar, it's often because different approvers have different tolerances and/or the characters are on the acceptable side of borderline, sometimes only barely.

Are there other franchises affected in the same way?

Yes. For examples, there's Warner Bros.' Marvin the Martian and Queen Tyr'ahnee as well as Toad from Super Mario Brothers. It was determined that the Martian characters looked too much like black humans to be acceptable and that the lack of mouth wasn't sufficiently non-human. Toad is frequently depicted as a toony human wearing a hat shaped like a mushroom cap rather than a mushroom creature. In the former case, Toad will get deleted while in the latter, he'll be accepted. After all, we're a furry art archive with a broader allowance for non-humans, but they do need to look the part.

Is it difficult to decide, sometimes? You bet. The guidelines over which characters are acceptable and when weren't made with the snap of the fingers. They've been heavily discussed before settled on, and even when we finally hash them out enough, we can still be fuzzy on the acceptability of some depictions sometimes. It can be rather difficult to decide at times.