2V0-642 - This is a self-paced course which will help you prepare for the certification
2V0-641 - This is the code of the certification itself that will be used to schedule the examination
642 is the current exam, based on NSX 6.2 : VCP6-NV (NSX v6.2) Exam
I did came to know that 0642 based on 6.2 but my real doubt is around the official cert guide. Official guide states it is based on 0641 but I see all reference to 6.2.
Digging a bit deeper:
NSX 6.2 was released in August 2015, the 641 exam was released the same month with the products are stated as "NSX for vSphere v6", and the official cert guide (which is specifically for 641) was published in August 2016.
So, it stands to reason that you'd see 6.2 referred to in the official cert guide for 641. I will edit my reference to the 641 exam in my reply above.
I'd suggest that as the official cert guide is for 641, it will include material related to NSX 6.0/6.1 rather than only 6.2 - so be careful if using it to prepare for the 642 exam.
Thanks very much for clarification.