Link to the website
The details are thus:
Top 8 voted heroes will be in a Voting Gauntlet (late Feb), and the winner is given out for free (mid-March)
In said VG, all units will score the same, so no need to worry if you don't have that unit for the 1.1x score bonus
Doesn't seem to be affected by color; it's possible, for instance, for all 8 heroes to be blue
Can't vote for regular Alfonse/Sharena/Anna, of course
It's not clear if there'll be banners for these top 8 heroes, but it's certainly possible, since last year's version of this event lead to a banner for the 4 winners
Edit: Midway results so far
For those who can't see the image, the list for now is:
Altina
Mythic Sothis
Legendary Azura
Winter Sothis
Lif
Edelgard
Legendary Alm
Legendary Leif
NY Alfonse Duo
Idunn
Brave Veronica
Surtr
Legendary Ike
Dimitri
Legendary Roy
Fallen Corrin F
Ephraim Duo
Byleth F
Claude
Fallen Tiki