You can take the JMT from the junction above the lower Vernal bridge to Clark point...then there's a connecting trail that goes down to the Mist trail above Vernal Fall. That's generally open in winter. (JMT from Clark Point to Nevada Fall is usually closed due to the ice cut).
Whether you can backtrack to the top of Vernal from that connecting trail junction, I don't know, but it's not very far. I'm not sure where the gate is that closes the Mist trail at the top of Vernal.
That section of the JMT up to Clark Point has some areas that get covered with snow, but you usually have a way to get around it.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo