How many other existing Champions are Vastaya?

None. They literally released a Q&A that clarifies that no one else is a vastaya except for the champs listed on the universe page http://universe.leagueoflegends.com/en_GB/race/vastaya/

http://na.leagueoflegends.com/en/news/community/q/vastaya-q

is the grim reaper was "blessed" by Vilemaw has vastayan ancestors and inherited some of their magical powers,but she is human.(udyr also has vastayan ancestors) is still a mystery,Rakan seems to think that he is a human with vastaya blood,but swain likely got his powers from a pact with beatrice. is something entirely different,though we have no idea what he is is an ursine,who seem to be spirits of the freljord is a dragon who was infused with human essence is a mutated rat is a demon is actually a vastaya,as she is listed on the vastaya-page

The ascended( & ) are humans who received celestial powers,they merely look like animal hybrids,but they aren't ones themselves Meanwhile looks like this because she was cursed.

Ascended are definitely not vastayas. They don't even carry vastayan blood.

None save for those on the lore page. Nidalee Swain and Udyr have Vastayan blood, but it's diluted. The rest are their own thing

Bro, Kindred is literally the grim reaper, it's not someone that just strolls from town to town, and pops into pubs for a beer before they go to the next town dude.