For prototyping purposes, the following white paper discusses methods and systems for analyzing MIMO signals: http://www.ni.com/white-paper/52382/en/. But since you linked to a VSA, will you be needing a calibrated instrument for your MIMO signal analyzer?
Regardless, if you are looking to build a MIMO test system, I would highly encourage you to call in and speak with someone to ensure you're getting the device that best fits your application and need: http://www.ni.com/contact-us/en-us
The PXIe-5668R could work but could also potentially be overkill depending on how you are wanting to analyze the signals. Besides that, there are also a lot of other considerations like frequency range, the need for synchronizing multiple analyzer channels, desired MIMO setup (2x2, 4x4, etc.?), testing a specific standard or creating custom waveforms, etc. If you are able to give more information on your application/needs either here or when you call in, that will help determine which device would work best for you.
Shalini M. Partner Development Engineer Alliance Partner Network National Instruments