Q.Snail Vine – not doing so well
I live in Palm Springs, CA and planted Snail Vine in large containers last June. It thrived through the summer and fall. However, during the winter it has lost some leaves and some of the remaining leaves have become slightly yellow. Should I trim it back in the hopes it will thrive once again? It is spring here in the low desert and many plants in my yard are perking up, but not the Snail Vine. Please help!

I think you are right in cutting it back to invigorate your vines. Snail vine is quite a vigorous grower, so it should fill in quickly.
I also would recommend fertilizing, a fish emulsion solution-half strength is my go to fertilizer.
I recommend once a week, until the vine has taken off. Then I would cut back.
Here is a link about you Snail Vine.
https://www.gardeningknowhow.com/ornamental/vines/snail-vine/growing-snail-vine.htm