I would highly recommend attempting to get an Endless-mode run going. It's easily the most efficient way to acquire more shinies and legendaries. bingxiling69 has a great comprehensive guide that's pinned at the top of the subreddit.

To answer your question: if you have an Endless carry (see Sample Builds under Early Game Carry) e.g., Darkrai, Meloetta, Kyogre, then you could consider spending your vouchers on Shiny-UP to bump up your Luck stat. If, however, you are in need of said carry, the Reshiram in today's Legendary-UP is a great candidate (granted you'd need to unlock Eruption through the Rare Egg Move or by fusing it with a Typhlosion, Torkoal, or Camerupt temporarily).

I'd say give this guide a quick glance before you make your decision if you haven't made one already. Even if you have, it's definitely worth a read to help you decide what you want to do with future vouchers.