I think the optimal rule is probably n+2 plains-cards for Dragon. It is less clear where Vista can be dropped to a 3x. In testing UWr Standstill, 3x Dragon definitely allows dropping a Vista, but unsure if 2x Dragon can also support a dropped Vista.

In terms of breaking the Dragon/Vista rules, I think you're probably looking at going for n+1 with 3 Dragon to 4 plains-cards and putting Vista down to 3x. Given Prismatic Ending is a better card than Disenchant/Path/Purge, consider 3rd color producer in SB. Unfortunately Triome enters tapped, making it a poor choice vs exile target Choke. Probably stick with a white/red or white/black dual, depending on what you want to accomplish.