Is liandry's viable as second item? third? thanks ^.^

on who? It's not that commonly built by a lot of people because there are typically better items. Not that it's bad, just not always the best for a build. I would say probably 4th item if you're going to build it.

Vel'Koz

I know it's viable on him due to the cc in his kit procing the bonus. I think --> -->then you could build it depending on what you need. If you just want raw data on it there is some on this site http://www.leagueofgraphs.com/champions/items/velkoz/middle/all

Lets say if your champion has CC , yes. If not get rylai first. Liandrys %2hp damage is increased to %4 when target get CC'ed.

on teemo it is

You need at least two forms of cc for it to be a good rush. If you do i'd say first or second completed item would be fine.

Many of the champion who abuse rylai's work well with it since the items have very good synergy together

etc.