It might have a poison stance toggle or interrupt or something like how Ninja used to. RIP Wasp and Viper Venom from Rogue. You are always missed Q.Q

But perhaps they're relating it more to how all Vipers are nocturnal and have special eyes that can see in a variety of light levels. Another job that works in darkmans, bene cove?

But more than that, it's just another Thief reference. In FFXI THF's best dagger WS was Viper Bite from about level 33 til level 61 or so. This is what Viper Venom on Rogue took its reference from, its first weaponskill being Wasp Sting.

Apparently someone liked the word and was like, "Another, flashier Thief? Let's call it Viper!"