venti_(genshin_impact) has only 47 posts right now which is meager for starting a tag discussion, but his associated copyright is growing rapidly and Venti's tag likely will too (though not as fast as the most popular female characters,) so let's nip this in the bud, and maybe set a precedent for other Genshin Impact characters since it has a number of male character designs similar to Venti's (e.g. the three other characters in post #4155426.)
Quite a few, though not all, of Venti's pictures are currently tagged as otoko no ko. Some obviously should be, like post #4152083. Others are just Venti in his default in-game outfit, like post #4136210. And others are ambiguous, like Venti in his other in-game outfit in post #4155423 and post #4155410 (I'm not sure this outfit can be considered either masculine or feminine, especially in the context of the game's setting, where it's his divine raiment as a god who has existed for millenia.)
Stepping back from Venti himself, I guess my question is: is there some factor other than crossdressing or an unambiguously feminine appearance/behavior marker that qualifies a character as an otoko_no_ko and justifies applying the tag to pictures of them? The otoko_no_ko wiki says this:
If you can correctly tell the gender with complete certainty from the thumbnail alone without looking at the genitalia, it's not this.
In the case of Venti I think this mostly depends on art style and the thumbnails usually make me go "weeelll looks more like a boy but could be a boyish girl" with less-than-complete certainty. The braids are certainly something that would make me think "definitely not a boy" if it were a real person or Western art, but in anime-style art I'd just chalk it up to unusual hairstyles for males being far more common. So I'm not sure what to do here, though my gut feeling is that Venti ought not to be tagged otoko_no_ko in most pictures. When we're not talking thumbnails or non-canon outfits, he's just a boy who looks like a boy.
(Also, it's largely irrelevant due to "tag what you see," but some story context about Venti and his appearance is here - this was a teaser trailer that came out shortly before Genshin Impact's official release, so YMMV about whether it's a spoiler. TL;DW: Even though Venti could be considered genderless, his appearance was always masculine, being modeled after a young boy.)