I've read through plottwist's reply and I also agree with his stance.
The most similar examples in other games are in LOI Leon can pair subweapons with orbs to create powerful (magical) attacks which have nothing to do with the VK. The orbs aren't useable because Leon has the VK, they're a separate thing altogether. Same goes for the spell books in HoD which are paired with subweapons. The only things Juste can do is alter the tip of the VK.
The DSS cards are dropped by Dracula's minions, maybe perhaps they harbour their own powers which manifest in the form of cards.
If these cards were specific to the whip then there'd be a slot system like materia in FFVII or something of the like which resembles this in the menu.
DSS cards also seem to resemble tarot cards which reference Astrology, notice the DSS Action cards range from planets Mercury>Pluto inclusive which is indeed reflective of the solar system. I'm not inclined to believe these are attached to the whip, because nothing in the game or the manual suggests this.
Why would I assume that it is? The Vampire Killer is off with the disappeared Belmont's or in the possession of the Morris clan and because in my head canon there is an awesome game that takes place before CotM set in 1820 which tells the story of Nathan's parents and Morris Baldwin's battle with an early resurrected Dracula where they create the Hunter Whip as a replacement weapon for the VK, an alternative method of fighting Dracula (like how they were looking for alternative methods of fighting Dracula in OoE).
Why would you assume it isn't when every previous game has featured the same whip pertaining to a Vampire hunter actually killing Dracula? There are only a few constants in the Castlevania games (not LOS) some of the main ones which are Dracula, Belmonts and the VK itself.
Eric Lecarde states in POR that Dracula is thought to resurrect in the year 1999, the Belmonts are not supposed to touch the VK until that time.
Someone's headcanon can be whatever it is, but Ecclesia is weak reasoning. Barlowe has the vessel to Dracula's soul and admits to Shanoa that his true purpose was to resurrect Dracula using Shanoa as a sacrifice(via Dominus) to destroy the vessel i.e. the order was a front for his resurrection, not his destruction. This was clearly explained in the game. Granted the prologue mentions organisations rising to stave off the resurrection of Dracula, but it doesn't go into detail and the prologue is later debunked by the ingame story in the context of Ecclesia.
If it is included in someone's canon that there just so happens there was another whip that could be used to defeat Dracula, I find this to be a stretch. Leon couldn't even defeat Walter with the Whip of Alchemy as powered up as it was until Sara donated her tainted soul to create the VK
If Walter's power was enough to best Leon (with the VK) then Leon would have died fighting Walter. If Mathias' power with Walter's soul absorbed was enough to kill Leon (with the VK), he would have killed him there and then. Walter could have destroyed WOA Leon but not VK Leon.
In addition, no "Belmont" or descendant in the canon timeline has defeated Dracula without using the VK aside from Quincy Morris who "kept him at bay" as per Iga's timeline, meaning killed him before he could resurrect in the Dracula novel
That's my issue with what is being proposed, there are contradictions and much simpler explanations like the VK/ HW going from:
Richter> Morris > Nathan> Reinhardt> Quincy> John> Jonathan> Julius
Rather than saying a brand new clan/ order of hunters was created etc based off information which is not well substantiated.
Occam's razor is getting thrown out the window along with the kitchen sink.
There is a case that can be made for Dracula not being at full strength in Circle of the Moon (and yes I am aware that it can be picked apart) - he was sealed at the start of the game by the previous events of 1820, he says that he isn't at full power and needs the rite which was initiated by Camilla using the combination of the full moon and Morris Baldwin, he says at the end 'I need my full power' before teleporting away into wherever the final battle takes place (after you have disrupted the ritual)... so for purposes of my head canon there is enough wiggle room for me for the whip defeating a not-at-full-power Dracula.
Head canon is head canon, not real canon, after all.
This is just how I like to view the game in relation to the series in the comfort of my own play through, not trying to convince anyone else to change their minds. The reason I suggest that for myself is because it has interesting story potential, and to me feels like something that ties kind of nicely into the prologue of Order of Ecclesia, and it doesn't get in the way of any details of the order of secession of the Vampire Killer whip if they are ever released, by side-stepping the issue. It's a different whip, it got lost to history afterwards. But hey, if it was revealed that CotM was part of the timeline and the Hunter Whip was the Vampire Killer, that would be cool, I wouldn't have a problem with that direction if Konami wanted to go there (I doubt they will be doing any revisiting of CotM anytime soon, however, as much as I would enjoy it). This is just my fun head canon, which I have been open about the whole time.
It's only by thinking about Circle of the Moon in relation to the canon games that any of this is an issue anyway - taken strictly as a side story and not part of the main timeline, there is nothing to suggest in game that any of the regular Belmont or Vampire Killer mythology exists in the universe of CotM. Officially, I would say that CotM and the Hunter Whip are just like your original comment on the matter, Zangetsu. As a side story it is just it's own entry into the series, and a very enjoyable one at that, IMO.
About Ecclesia though:
Also yes Ecclesia is about resurrecting Dracula... but it was set up 'publicly' to try and defeat Dracula, and the prologue mentions many organisations made with the same goal, but were dissolved when they couldn't deliver the goods. That part of the prologue isn't debunked, nothing about Ecclesia being a front for resurrecting Dracula means that other organisations and other counter measures weren't tried out as well.
About Ecclesia - i still don't see how Ecclesia being created to bring back Drac
'statue' of Haunted Castle Simon in Medusa's lair in OoS, if that gives you any more fodder for your theories.
On OoE - I think it's safe to assume that the other organisations are exactly how they were presented in the opening - other organisations that tried and failed to create countermeasures against Dracula. Otherwise it is just over thinking things, IMO. Sure, if they ever explore that plot point they could expand them out to be doing whatever they wanted, but as it stands, I see no reason not to take that at face value. Organisations tried, they failed. IMO, it isn't 'wishy washy' as to what they were up to - they were trying to find ways to stop Dracula, they just couldn't.
Head canons have been based on far flimsier pretenses so I am not very worried ;)
Speaking of head canons, your reference to the vessel Dracula's remains being sealed in during OoE as being the Crimson Stone is a theory which we discussed once before, in another CotM (and OoE) based thread (where I was discussing how I thought that, after starting OoE, it actually helped make CotM feel MORE like it could be part of the main timeline, to me).