Vivi should just stay in FFIX.
In any case, Sin maybe brings Vivi to FFX if Vivi becomes a dream, like you can teleport between Terra and Gaia by riding Invincible or how you travel to Crystal World. Apparently, Vivi (black mages)remains in FFIX, but his soul (dream/emotion/whatever) is brought to FFX. well, if Sin works with Lifa Tree, nothing is impossible. Sin is but another means of transportation, Lifa Tree provides Sin with the sources of energy like mist.... Such a cool idea! JK,JK