The Nvidia 1080 will not go down in price when Vega is released. Some months ago Nvidia became aware of AMD's next generation video card and made an educated guess as to it's performance. Nvidia responded by releasing the GTX 1080 Ti earlier than planned and lowered the price of the 1080 at that time.
So, the price has already been reduced, and Nvidia will not lower it again. There are other factors in the equation, however, none of them will cause Nvidia to further reduce the price of the 1080.
I began my career in the computer field over 30 years ago and have found there are many factors that influence hardware pricing. It is impossible to give you a prediction with 100% certainty about your question, will Nvidia lower the price of the 1080 [again], however, the indicators point to a 99.9% probability that Nvidia will not lower the price after Vega is released.
Edit: As to the mini version of the 1080, if you purchase it find one with a good cooler. Without proper cooling the card will throttle if it gets too hot. So, card design is important as is case cooling. The only reason to get a mini is if you have limited room in your computer case. Make sure whatever card you purchase will fit. Some mini 1080s are too large for an Mini-ITX build, however, is should fit Micro ATX. Measure how much room you have.
Also, do your research well because there are other downsides, such as extra noise or coil whine in some models.
Here is an example:
http://www.guru3d.com/articles_pages/zotac_geforce_gtx_1080_mini_review,37.html