Sry if this kinda sounds like i'm clueless or something but what were Zexion Lexaeus and vexen going to do with Riku?

Use his darkness in some way.

If I'm not wrong, they were planning to use Riku to challenge Marluxia and Larxene. After they were destroyed, Vex, Lex, and Zex would do away with Riku.

 

Needless to say, the plan didn't go well.

i think they may have used it to toy with soras memories

They were going to use his power of Darkness. (Does anyone else think it's kind of wierd when you're battling him and he'll go all "Power of Darkness!"? It sounds so dorky!)

Yeah, they were going to use him to elimnate the traitors(hence Zexy's line to Axel: "...We were going to set him off against the traitors, but with Marluxia gone, there is no need. He is nothing but a nusiance now.") and then trash him. But wasn't he also used to help perfect Vexen's evol replicating powors?