Here’s how it works. The great vault rewards content based on a % of content that you do. So if you do, as you say 14 keys at one level (17+) and 1 key at a different level (3+) your choice of rewards will all come from the 17+ loot table.
Same with raids. If you do 9 bosses in Mythic, then one boss in LFR, you’ll get mythic raid choices from the great vault, and no LFR pieces.
The reason they’re doing this in particular is to avoid situations that are exactly the opposite of what you’re describing, with one person doing a single M17+ and then filling the rest of the dungeons with M3+ and getting loot that is from M17+ dungeons in the vault.