Good call with Grant/ Hammer Shiroi, beat me to the punch
About VK's fading power, VK still held power even in the Belmont's exile during Leon's time to Trevor's time, which, (not considering Sonia cuz she's non-canon) the power did not fade away, actually, it became stronger. Also when the Morris' had the VK (Bloodlines and PoR), it was still as effective as. Even en Bloodlines, Castlevania was in its ruin then. Thinking that the Dracula is still around and dark power still lurking (like Celia) (also that Castlevania is technically still existing; just in the solar eclipse. If it was gone for good, we wouldn't be seeing AoS anyway) , I really don't think VK lost its power in any way.
I think you're misunderstanding my comment in context to a few facts:
Re: LOI - The Castle in LOI (although unconfirmed) is not considered to be the original Castlevania i.e. the one seen in Castlevania III. Because the VK was conceived at the time of LOI and Dracula didn't have 'his own Castle'. This would not apply. The VK's power (rage) could have faded after CVIII and become completely active again by CV1.
-Re: Bloodlines - True that when the Morris clan during Bloodlines had it, it was at Castle Prosperina, though Castlevania was never resurrected at this time and the game was made a long time ago.
-Re: POR - The Morris/ Lecarde descendants only unlock it when Castlevania has been resurrected once again.
-I never said the Vk was "gone for good"
-I never made the assumption the Vk's "power faded", I only assumed that the VK's power is its rage; Sara's rage from LOI.
This is not an assumption as it is said at the end of AOS by Julius, I even recall a thread discussion (which I believe I started) where Shiroi translated this line in Japanese, and it was basically the same.*
*I reiterate,
not saying the VK's power is completely gone, simply that it's rage has been quelled temporarily.
However, the Castlevania<>VK link is simply a theory. Given the context of what all the games have in common, it's Dracula's potential resurrection, or more specifically, the resurrection of the Dark Lord (which also covers Soma). Because of this, the VK's rage would still be at max during the events of DOS. Hence, why I stated initially prior to my theory that Julius was nerfed so hard to give his mode more difficulty - his super jump being nerfed is evidence of this, and the proof being that in Soma's mode Julius outright scales the entry to the castle with Yoko commenting that he was able to do it because, well "He's Julius"
Also, Julius' age don't matter, cuz like in AoS he was a kickass, and in Adventures, Christopher was narratively stronger then Soleiyu (or that is at least how I type it). Thinking of that, I don't think age doesn't matter for Belmont's thousand year-old vampire hunting. (We can say about Eric or Morris Baldwin, but they are nit Belmont, so hey ho!)
I literally never commented on Julius' age