I've managed to boot openwrt with an alternative version i had with a tp-link style theme and after that i could boot the newer version. (i don't know what did it but it worked)
It's possible that you use wrong vendor firmware, isn't it? Try older/newer version.
Regarding firmware i always double check the versions i download so i don't have the wrong ones laying around.
You can try prepare new image from vendor firmware for flashing from openwrt command line. (...)
I've never messed with stuff specifically like this but i'll give it a try in the next days, i'll keep you posted.