This is a very simplified response, but I hope this will feel useful:
Vedic Astrology is centrally geared to our incarnational life purpose, karma and its unfoldment.
Western astrology at its best gives deep psychodynamic perception and possibility for transformation, and understanding the impact of the energies of the outer planets discovered since the formulation of Vedic astrology is also a must: Chiron, our existential wound in the healing of which we can become the healer of others, and the outer planets (the Gods of Change): Uranus, Neptune and Pluto
Note that, generally speaking, the sidereal zodiac of Vedic Astrology is far more sure and accurate and powerful than the tropical zodiac of western astrology (but of course a lot can be said about this issue).
I specialise in astrology readings combining western and vedic astrology. See: http://www.starwheelastrology.com