How do you start it? and is there any way to start it through the console???

Romances will continue in ToB, but IIRC they can't start in it. You can only romance her if you're a human or half-elf. Um, and it start like any other romance, through dialog.

You should be able to find a way, have a look at the global variables in shadowkeeper if you like or you might be able to use the console, check the walkthroughs and romance guide in the BG2 section. Hope this was useful,

Cheers, Void

The romance with Viconia triggered with my Half-Orc aswell...