Ironstylus, Lonewingy: I am really excited to hear that seductive vampire is being considered. This is the theme that I connect to Evelynn and is the only one that I believe that makes sense for a cohesive and clear thematic package. I have been wishing for this theme to be chosen since her VU has been up for discussion
Currently, LoL is really missing out on a female vampire theme. Vlad to me is a hemomancer and represents the mage like aspect of a blood wielder, reinforced by his kit. Evelynn however, has the possibility to represent a stealthy and relentless, magically influenced champion/kit that encompasses the other spectrum of the vampire mythos. I can see how some people are complaining of potential overlap but the possible similarities between a vampire Eve and Vlad but this will ultimately make Evelynn feel like she is appropriately placed in the LoL universe in a manner similar to other related champions (i.e. MF and gankplank, fizz and nami, etc.)
Furthermore, some of the only highlights in Evelynn's design come from her vampire like parallels. Her masquerade and tango skins connect with a vampire sense of elegance/atheistic which I think is a theme that has more depth to it. Her default skin and shadow skin reinforce the BDSM look which I believe are distasteful and bland (purposeless sexiness is interesting for 0.5sec as it is trope done to death in video games). I think it is important for Evelynn's sole purpose to not just be "sexy." Sexiness as the primary theme is very flat because it doesn't represent a believable character trope. Sexiness in reality is best established when it is connected to a purpose or agenda. Examples include Selina Kyle/Elise (sex appeal for manipulation and goal seeking purposes) or Medusa/Cassiopeia (sex appeal as a juxtaposition of her curse for her vanity/ambition) or MF (sex appeal to highlight reckless abandon and femininity in a historically male dominated role as a pirate/ship captain). These characters have depth and are believable, respectable and therefore fantasy archetypes that people want to inhabit.
The Succubus trope, when primarily emphasized, is one that I have never found to be effectively executed. The succubus mythos is one that traditional involves seducing men to propagate demon children. The only part of that fantasy that can be incorporated into a video game visually is "demon" and "seducing" so the final package is a demonoid that is scantily clad. Let's take two popular examples, one non successful one (Queen of pain from Dota 2) and one successful one (Kerrigan from SC). QoP is non cohesive fantasy package. All of her in-game quotes are really about being a queen spoken in an overly sexualized way. None of her abilities have anything to do with her fantasy except for the idea of "screaming" and visually, she is neither regal nor sexy, just a woman with wings in a bikini. Kerrigan, however, is a succubus variation (even has a succubus skin in HoS) but everything interesting about her comes from her past as a human and her role as a queen of aliens. She has depth of as a character, is a respected ruler and is an exciting icon villain. Her sexiness is secondary to her own motivations and is visually balanced by her disgusting alien features. It is that exact balancing that Evelynn would be able to achieve if she was a seductive vampire and not a BDSM succubus.
As Vampire, Evelynn could embody a champion who is enigmatic, mysterious, evil, reckless, remorseless. Her sexiness could reinforce a "shadowy elegance" and her role as a "creature of the night" who has little regard for how she is viewed by others. As a vampire, Eve could draw reference to iconic fantasy characters such as Isabella Von Carstein (Warhammer), Elizabeth Bathery or Carmilla (castlevania). She would develop a complete conceptual package and a the fantasy experience that LoL continues to strive for with their character designs.
Please, choose a vampire directionality for Evelynn. Lonewingy's work on nami is some of the best artwork and representations of a mermaid currently available in pop culture (That ruffled fin blouse is brilliant/CFDA award worthy). I could only imagine what she could do with Evelynn as a vampire.