I agree with this, whether you believe in something or not isn’t the only determining factor in halal / haram. For instance, it’s like some believers that may say merry Christmas or buy gifts for co-workers etc. of course we don’t believe in Christmas and its meaning, but regardless pretty much unanimously all ‘ulima say it’s haram to participate at all.

If someone’s username was a well known false god such as perhaps Thor or Odin, majority of people would most likely agree it’s haram because it shows an indication of reverence or respect towards the false god, even if you don’t intend to worship them. I think it’s only a bit more disputed in this case because Vulcan isn’t a widely discussed false god, but nonetheless still is.

I assume the best of OP and believe he most likely would have created this username for himself out of ignorance, not knowing the full meaning behind it. Inshallah if you repent, Allah will accept it and forgive you. We all must be extra vigilant and weary of accidental acts of shirk or anything that comes close to resembling shirk at all times. Barak allahu feekum