Unfortunately, amazon instant video is not available on your particular model.
It may have been previously (when new), however the marketing contract between amazon and vizio (as well as many other brands) that covered this model has expired, and is not expected to be renewed. You can read more about that here: http://heavy.com/tech/2015/09/amazon-instant-video-no-longer-working-streaming-on-google-tv-android-samsung-lg-vizio-sony-hisense-netgear-chromecast-fire-4k/
Your next best option in this case is to connect an external streaming device instead, such as a roku, fire tv, htpc or smart bluray player and get all the same functions and many more, with better performance than what your tv has now, along with better support as well. Of these, the fire tv box or stick will be the most capable.
Best regards, ***** ***** for the news,
I am an independent technician and I've spent my personal time, unpaid, in an effort to assist with the question for which you solicited my services. This site works on the honor system, and to that end, I have prioritized working with you ahead of other paying customers.
Although you’ve already paid your fee to the site, I am only compensated for my efforts when customers such as yourself leave positive feedback for my work. Again I apologize there is not a better outcome here, though this is really beyond either of our control. I do hope you’ll consider leaving a 5 star rating for me, rather than “shoot the messenger”, so that I may get credit for my time spent in good faith to answer your question accurately.