idx = vectorLayer.fields().indexOf('fieldName')
values = vectorLayer.uniqueValues(idx)

>>> set(range(4) + range(5) + range(6))
set([0, 1, 2, 3, 4, 5])

values = set()
layer.select()
for feature in layer:
    index = layer.fieldNameIndex(ATTRNAME)
    values.add(feature.attributeMap()[index].toInt())

values = set()
for feature in layer.getFeatures():
    values.add(feature[ATTRNAME])