Yeah, Q works pretty well with Liandry's, and so does R, but look at it this way
As a Vel'koz there are much better items to get
Sorc boots - huge early power spike, very cheap, MRpen is Vel's strongest stat
Guise - again, MRpen and it's very cheap for its damage bonus. Upgrading to Guise requires tons of gold and you get only low AP and burn, burn might be good but AP just sucks.
Luden's - increases Q damage by like 40%, strong synergy with MRpen
Void staff - more MRpen, with sorcs, guise and runes allows dealing true damage to champions with <80 MR
Support Vel goes Sightstone like 2nd or 3rd item, depending on situation
Mid Vel usually goes for Morello CDR and regen
Usual full Vel's build looks like this
Sorcs
Luden's
Liandry's
Morello
Void staff
Frostfang sightstone/zhonya's/utility item
So while Liandry works, there are items with much higher priority. So its good but its not that required or core item.
As for Zyra and Brand, they are excellent Liandry users, both are pretty decent supports with tons of collateral damage. Liandry+Rylai is very good here.