Maybe it is
our job to clarify what the "omnipresence" in Hellsing actually means. I have a feeling the "omnipresence" in Hellsing is not true omnipresence as we can see in other verses. Rather, just "very good teleport with a quick thought" plus "very good enhanced senses".
Also, recognising how regeneration works for Alucard then it is just combined factors of raw power and time that the "three million souls" of Alucard runs out.
Anyway yes
Dio Brando vs
Alucard (Hellsing) will be at least interesting in debate and fun to watch for a simulated fight. And a good cash-in matchup.
Now, does anyone feel that Rooster Teeth tend not to use a franchise from a lore whose franchise owner (or even owner) is (1) ill-defined or just (2) unknown or (3) comes from a "shared creation" or (4) public domain or (5) the copyright owners and successors are all dead? This may potentially skip them from the franchises of "real" mythologies, "too ancient classic literature" (like the original Journey to the West or Chronicles of Narnia), and franchises like SCP Foundation and Cthulhu Mythos?
Maybe. Or maybe the new technique is deemed by them to be a non-factor for whatever mind in their heads.