How Vastu is different from Fengshui? — It is upto each practitioner to decide which science is more relevant in Indian context but a deep understanding of both sciences is a must for every competent practitioner. What one needs to understand here is that like Vastu Shastra there are many healing techniques developed around the world having vastly different techniques and remedies which make perfect sense in their own context only. An indiscriminate mix-and match of different techniques without any in depth knowledge can defeat the very purpose. There is no contest between Vastu Shastra and other methods like Feng Shui, this is about indiscriminate mixing of different systems. It is important to understand the basic essence of remedies for them to work. For example, symbols are extensively used in Feng Shui. Symbols work because they from a part of the collective consciousness or collective psyche of a class of people / a race or a nation. The symbolism is usually rich with nuances and contains multiple shades of meanings.
The power of symbolism is great because so much is conveyed and communicated at a glance. It works because the power of generations of traditional / folk beliefs and religions is behind it. To give an example, the dragon may be considered an auspicious animal in china but in India, it may be perceived as a ferocious fire emitting snake. In this case, I fail to see how a dragon symbol can add any positive effect. Any fact, figure or statement without reference to the context can never make sense. In Indian context, whenever we state Laxmi or Kuber it automatically refers to wealth and prosperity in the Indian mindset. Same words may be meaning less is Japanese or Chinese context. Same way a fish aquarium creates a picture of wealth and prosperity for Japanese but if it is placed in a Jain home in India as a remedy it may have a reverse effect as it goes against their deep-rooted religious instincts giving them a guilt complex.
On the brighter side the intelligent use of mirrors and wind chimes, some of Feng Shui remedies can solve many a tricky situations. Both these Sciences have a touch of Mysticism, as they incorporate the philosophical thoughts of their respective origin. Also, the climatic conditions of the two countries favour different directional preferences. For instance, Feng Shui favours South as an entrance direction, a high rise towards North and South East for water source. Vastu, on the other hand, puts a premium on an open North, a high rise towards South and North or North East for water source.