I've seen my wife tickled on her feet by three different women, all vanillas with long-ish nails that they intentionally used. One of them, my wife's best friend has tickled her multiple times over the years and I've seen her try using her fingertips instead of the nails and then realizing the nails worked better in realtime.
I also witnessed the same friend use a brush that I use to scrub my hands after work to try and get ink runoff from a tattoo off the bottom of my wife's foot. She knew how much it was going to tickle my wife for sure and she kept taunting her by saying "what's wrong Keri?" and "does it tickle?" while she scrubbed. I don't think the idea to use the nail brush would have crossed her mind if Keri wasn't trying to get the ink off, but she saw it in the soap dish and grabbed it.
Vanillas that tickle are fascinating to me for a lot of reasons, technique being one of them. After witnessing a tickling I always wonder what made the person go for it. Sure, a bare foot or exposed belly is always a trigger for me to think about tickling, but I have a fetish. The person who doesn't and sees a bare foot and goes for a tickle--what's going through there mind?