A Rg Ved samhita (10.98) composed by Devapi, mentions Shantanu ruling Hastinapur. MB confirms that Shantanu's elder brother Devapi gave up his right to the throne in order to persue his philosophical intrests. Yajur Ved mentions no one younger than Dhruturashtra. There is no mention of a king after MB in the Vedic samhita.
The conclusion that Vedic Samhita were complete before MB War is also supported by tradition that Sage Vyas had organised the Ved. He wrote the MB after he had completed the giagantic work of organising the Ved.
Recent archaeological discoveries also support the above conclusion. Archeology proves that the Sarasvati river ceased to be a sea flowing river by 3500 BCE, and had dried up around 1900 BCE. Older portions of Rg Ved praise the Saraswati as the best river. Thus those cannot be dated later than 3800 BCE. The latest book X of Rg Ved only mentions Saraswati among the list of rivers. This must be attributed to a date later than 3500 BCE, but before 1900 BCE . MB belongs to the same period. It describes Balaram's journey along the dry banks of Saraswati and the flow of Saraswati upto Vinasana.
Chandogya Upanishad (3.17.6) mentions that Sage Angiras taught of soul's immortality to Devaki's son Krushn. And, this is exactly what Krushn taught Arjun in Geeta. [Bheeshm Parva] This implies that the Upanishad was being compiled during the MB times.
Shulba sutra, the book on 2D and 3D geometry, is assigned a date before 2000 BCE . It reflects the knowledge of the SS people that was used for town planning and building in the SS valley. If SS & MB were contemporary, this Sutra could also belong to the same time frame of around 3000 BCE. 
Core Puraan later expanded into 18 Maha Puran and 18 Upa-Puran. The heroic deeds of many kings described in Ved are only mentioned (and some not even mentioned) in the Puran's. eg: Sudas, Divodasa, Somaka, Abhyavartin chyaman. This can be explained by the fact that the Puranic geneologies were constructed long after the ved. So long after, that some kings were forgotten. Even the heroes. 
Agam's like Pancharatra are mentioned in MB. Ayurved is mentioned in Mahabharat and was a developing science during the MB times.