Harmony of Dissonance --which is as much of a direct sequel to Simon's Quest as we'll probably ever get, having gone out of its way to establish Juste as Simon's grandson and confronting him with Wraiths that look like the decayed zombie of his grandfather while once again the Relics of Dracula play a huge part in the story-- very clearly establishes (albeit through gameplay) that the Vampire Killer can be altered with various tips on the end and magical stones inserted into (I guess) the pommel. As a strictly gameplay feature, this didn't mean much to me until the feature started cropping up in other games enough that I took notice. Trevor Belmont could do it in his (noncanon) mode in Curse of Darkness, but even in the actual canon story, when Trevor wields the whip, it glows in a manner suggesting his gameplay mode's alterations, which leads me to think there's actually a link there and that while Trevor Mode isn't canon, his ability to alter the whip was, which could suggest Juste's ability to do so might actually be an in-universe thing.
So if Simon's ancestor could do it, and his grandson could do it, there's zero reason for me to think that Simon couldn't do it if he had the tools and know-how to pull it off.
The best part is that Harmony of Dissonance, through that same feature, implies that Christopher Belmont actually possessed a relic that allowed the Vampire Killer to literally fling fireballs. Badass.