In The Wolverine, is the motivation behind Viper's involvement in Yashida's immortality project ever explained? I can only recall her mentioning that Yashida hired her.
I can only recall her mentioning that Yashida hired her.
It might not be worth a complete answer since it's just speculation, but I'd consider that motivation enough for her. Yashida is extremely rich and goes out of his way to overcome death, so it's likely he paid her quite a good amount for her services. Unlike Harada, Viper doesn't seem like a person whose loyalty is driven by higher ideals than mere payment.
If she had really just been the doctor she was introduced as, you wouldn't ask for her motivation either (well Ok, you could say doctors have an iherent motivation to heal people, but they usually won't do it for free either).