This post is honestly way too late, but I only got the recipe recently as my main scribe has been sitting at level 50 the entire expansion.
Raid consumables are always popular
Vantus runes are a special type of raid consumable that you can use one of per week. It increases your versatility when fighting a specific boss. Anyone still progressing in Mythic will want to use these, and even some heroic raiders will want to use it.
Getting the recipe
The recipe can drop from any boss in Castle Nathria on any difficulty. This means you need to get to level 60 and get at least enough gear to queue for LFR. I personally got it on my first boss kill after leveling inscription, so the drop rate is presumably quite high. If you can queue as a tank or a healer it will obviously be a lot faster, but either way it does not take long.
Crafting cost calculations
The Vantus rune requires 10 each of Umbral and Luminous inks, as well as 1 Tranquil Ink. Since it requires equal quantities of umbral and luminous ink it is easy to calculate the crafting cost based on the required number of herbs, specifically Death Blossom and Nightshade.
Based on my milling ratios you will on average need 2.29 Nightshade and 62.86 Death Blossoms per Vantus rune. When buying materials you should buy about 30 death blossoms per Nightshade to get the right ratio of pigments.
We can use the nightshade and death blossom quantities directly in the TSM setup to get correct pricing. Since it will generally only make sense to craft if it is profitable to mill and make inks it works well. Inks generally sell quite badly on their own as well.
For posting you want to post a small quantity at the time and repost often. You can cancel, but just keeping a stack or two and posting 1-3 every 5 minutes works as well, and takes significantly less time. As usual with consumables the raid reset day is the big one and resets will often happen, so be ready if the price spikes.