Summoner Question: Where should I spend my Myth tomes first?

[Sleigh]

I'm trying to figure out what is the best gear set to start Coil: Vanya or Darklight? (as a SMN)

I know that Vanya full melded set very expensive, but so good.

You can very reasonably get through turns 1 and 2, if your team is good, while using full Darklight. It won't become apparent DL is a little low for BC until you get to Turn 4, so you might want to see about how you feel you need upgrades if/when you get to turn 4.

While Vanya is a significant upgrade over DL, it's mainly due to Critical Hit Rate and Determination. Both are important to DPS, but you need high quantities of them to notice a difference, especially compared to INT - from which you'll notice a DPS boost every few points.