Sure. The Roland EA7 doesn't have a vocal harmonizer built it. It isn't essential but I do like to have one.
The VK also processes your voice automatically, adding e.q., compression, gates, etcs., but only as needed. It is simple to use. It has Reverb, Echo, Pitch Correction and Feedback Protection. It's really a neat outfit. You can control what harmonies it sings by midi from the keyboard, or from audio. In others words, plug and play!
I already have a similar unit, the Harmony M, which is no longer in production.
I thought it would be easier to sell the latest greatest, which is the VK, than a 10 year old unit.
2. If you are coming directly from the keyboard, you will need a cable that has two 1/4 inch jacks on one end and a stereo mini plug on the other. If you are using a mixer in addition to the keyboard it depends on which outputs you are using as some are 1/4 inch but others may be 1/8 inch or mini size.
These cables are readily available. I can send you a link to one, but I'm on the way to work right now.