Right now I'm 8 for 8, so currently 100%. Before I started crown farming for platinum a few days ago, I only had about 2 Large Crowns and 4-5 Small. A couple vouchers a few days ago and I got BOTH trophies (10/10).
Today, 3 more times, I got a total of 6 crowns. I'm so far averaging 2 Crowns per Voucher.
It's not concrete. It's possibly a coicidence. It's not "false". And I take screenshots of every crown.
It's something I am testing on my own accord but since I saw this post, I thought I'd share my findings.
Besides, "double" zenni... Lucky Vouchers can also possibly be used as a "double everything" (zenni, rewards, crown rates??)
But this testing isn't concrete yet until someone else tries. I mean, what else you gonna use your LV's for? Try using a Lucky Voucher when doing a 5 monster event and reporting back to here.
I will resume testing tomorrow morning (12 hours from now) -- and report my results here.