I have to admit to never having heard the term "vanilla fag" or at least considered that much, but you have to consider that things like "virgins" "incels" and even "NTR" are tropes.
The reality can be different and a well-written story should diverge from these tropes in order to give characters depth.
Real people may be virgin or incel for different reasons than you imagine.
Not being a manga fan, I have real difficulty understanding the Engrish in the plate in the OP, but it seems a bit shallow. On the other hand the premise, secretly liking a girl who you think is "out of your league" but not daring to ask her out, is I think understandable for many male adolescents. It's just one of the problems of adolescence where you have desires but no self-confidence or skill. But many girls have a similar problem: they want to have a boyfriend but not become a slut. The threat of NTR (ie. some other guy getting the girl the MC secretly desires) could motive the guy to act but IRL they still would likely be too shy.
But as I said, that is part of normal adolesence. It's unfair to call the guy an incel, even if it technically correct. It's also not correct to call it NTR because there is no established relationship outside the MC's head. On the other hand, you also need to give the girl some credit: IRL the chance of her going out with that other guy is also not that big and if they are, the MC simply needs to accept it was not meant to be. And there is no shame in that: if I walk on some beach in the summer, I see plenty of good looking girls but I have rights to none of them and the actual chance of fucking even one of them is pretty small, regardless of my looks, confidence, age or money.
I feel like we have difference view on incel here, like who should be being call incel and who shouldn't, in your example above, I would never call guy like that an incel, real incel on the other hand, by default they want the girl they secretly like attention all for themselves, childish but understandable, until they call that girl a whore/slut when they learn she had sex friends, or even she just interest in many guys, they find some excuse like "she'll cheat on me if we are in a relationship anyway" when they back away, but if the incel see another guy still chase that "slut" girl after he learns about her "nature", the incel will call him a simp.
And this is the most common thing about incel I have seen so far both online and irl, they want girl they like to accept who they really are physically, mentally and financially, therefore they never go to gym or do anything to improve their image in the opposite sex eyes but the idea of having a girl approach them are their ultimate fantasy lol, even I back in middle school, I was still a fat ass I have girl ask me out, it's just the incel horrible personality and their shitty view on women make them stay virgin, being virgin is not a bad thing obviously, I know many people have anxiety due to their lack of inexperience with the opposite sex, I never slander someone because they are virgin, but if they are virgin and talking shit about how women should be and slander them when they act against that idea, then I will make exception.
When it comes to fiction, I wouldn't expect everyone are able to enjoy NTR, I myself once scare of reading and NTR, checking tag to make sure before reading even it mean to spoil the story, but I never go out of my way to insult people who enjoy it or make it, artist have total creativity over their work and I'm sure people with common sense can understand this, vanilla fag on the other hand always feel the need to shit on anything remotely relate to the idea of heroine once have interest in anyone before they and the mc are in a relationship leave alone NTR, vanilla fag can't stomach anything beside the idea heroine love them for who they are and for some reason want to jump on their dick and have their baby, sound familiar?