[needs-docs][processing] Respect user set default vector output format

in vector split algorithm

The created outputs will now be created using the format specified from
the Processing setting default vector format setting.

python/plugins/processing/algs/qgis/VectorSplit.py

@@ -29,11 +29,11 @@ from qgis.core import (QgsApplication,
                        QgsProcessingParameterFeatureSource,
                        QgsProcessingParameterField,
                        QgsProcessingParameterFolderDestination,
-                       QgsProcessingOutputFolder,
                        QgsProcessingException,
                        QgsProcessingOutputMultipleLayers,
                        QgsExpression,
-                       QgsFeatureRequest)
+                       QgsFeatureRequest,
+                       QgsVectorFileWriter)
 
 from processing.algs.qgis.QgisAlgorithm import QgisAlgorithm
 from processing.tools.system import mkdir
@@ -88,6 +88,11 @@ class VectorSplit(QgisAlgorithm):
         fieldName = self.parameterAsString(parameters, self.FIELD, context)
         directory = self.parameterAsString(parameters, self.OUTPUT, context)
 
+        output_format = context.preferredVectorFormat()
+        if not output_format in QgsVectorFileWriter.supportedFormatExtensions():
+            # fallback to gpkg if preferred format is not available
+            output_format = 'gpkg'
+
         mkdir(directory)
 
         fieldIndex = source.fields().lookupField(fieldName)
@@ -104,7 +109,7 @@ class VectorSplit(QgisAlgorithm):
         for current, i in enumerate(uniqueValues):
             if feedback.isCanceled():
                 break
-            fName = '{0}_{1}.gpkg'.format(baseName, str(i).strip())
+            fName = '{0}_{1}.{2}'.format(baseName, str(i).strip(), output_format)
             feedback.pushInfo(self.tr('Creating layer: {}').format(fName))
 
             sink, dest = QgsProcessingUtils.createFeatureSink(fName, context, fields, geomType, crs)