The major difference between Vastu-Purusha and Vishwakarma is that the latter is renowned for the construction work done for the deities and he has a definite worldview.
All his projects are grand projects. This includes the construction of the world, large expansions, townships etc.
The latter is responsible for the architection of each and every bit of land within the premises. The laws according to which Vishwakarma acts are based on the concept of Vastu-Purush and his Sub divisional-configuration (Pad-Vinyas).
Vastu-Purusha is also omnipresent and his insrtuctions guide Vishwakarma in his constructions.