Feta can be quite salty, so taste the vinaigrette before adding any salt.

3 T sunflower, or similar, oil
3 T extra virgin olive oil, or use all olive oil
1/4 cup finely crumbled feta cheese, or to taste
1 T apple cider vinegar
1 T red-wine vinegar
2 t lemon juice, or to taste
1 freshly ground pepper, or to taste
salt, to taste, optional

1. Put all ingredients, except salt, in a blender and whizz until smooth. Taste for seasonings, and add more of any of the ingredients (plus salt, if you think the vinaigrette needs it) to achieve vinaigrette nirvana.