Yama has no flames In bankai just very hot an strong spiritual pressure.Mmm no, just no.
No you still don't understand, Toshiro freezes everything to an absolute zero(No mater no molecules move no energy to spear its static everything) and no matter how hot Absolute zero is absolute zero, and Yamamoto cant unleash his flames because there is no energy to release.
So the end result is toshiro will be vaporized.