Links between words (when a letter is carried across to blend with the next words) are sometimes a personal/regional choice. vienn ni / vienn ti is like (if you know the song "Let's called the whole thing off") potato : potay-toh potah-to (if you don't know the song, here is a link http://artists.letssingit.com/fred-astaire-lyrics-lets-call-the-whole-thing-off-dksxl8n )

In normal conversation, like you said: "vien i" And you will do an "enchaînement", so you'll pronounce it: "viè ni"

vienne-ty so the last "t" of "viennent" is pronounced and formed a liaison with "y" :)