The "buy MSP 13 + get 14 free" special offer which was extended a couple of times is not valid any more AFAIK. So I don't think you'd get v.14 free, if you buy v.13 now.

This article should give you a good idea of the differences between MSP and VPro. It's a bit outdated of course as it's based on vs.12, but since then MSP 13 has basically only added some of 4K options and not much else of Pro features. This preview of MSP 14 says that it "will have a powerful multicam editing workflow." Read that "sneak peek" for more.

The current product comparison links are below. Note that the MS table is for v.13 at the time of writing.

http://www.vegascreativesoftware.com/us/vegas-movie-studio-product-comparison/#productMenu

http://www.vegascreativesoftware.com/us/vegas-pro/product-comparison/#productMenu