If you read the terms on vx.com:

"Plus, make your status points go even further, and jump start the new year by carrying forward any additional status points you’ve earned above the annual qualification requirement."

"The Elevate status qualification year begins on January 1 and ends on December 31. If the Status Points posted to your Elevate account in any qualification year exceed your earned status threshold, the Status Points will be applied to the following year's status qualification. Status Points are not redeemable for Reward Travel."

Both above suggest that if you didn't earn 20K for Silver by 12/31, then nothing will carry over as you have no status points earned 'above the annual qualification requirement' in the year