I have a berry wine, stuck at 1.026SG and with a high malic acid (quite sour). Should I try to restart the fermentation by adding fresh yeast? What about yeast from another vat? Is there enough sugar still in the vat to continue the ferment?
There are specific steps to restarting a stuck wine/mead/high gravity beer. You can read the whole procedure here. You'll need some champagne yeast or some other special yeast used to restart stuck fermentations. But it starts with this:
For restarting 5 or 6 gallons, take a quart jar and fill it half way with the wine in question. Add to that, water until the jar is 2/3 full. Put in the mix a 1/4 teaspoon of yeast nutrient, and 3 tablespoons of sugar. Be sure that the sugar becomes completely dissolve. Now you can add a whole packet of the Champagne yeast. Cover the jar with a paper towel and secure with a rubber band.