#but once meryl gets captured then aha trimax vash is personally confronted with how much he can't afford to lose her

[the-nysh]

Oh sweet! Looks like Trimax vol4's when Vash actually starts calling Meryl by name on a regular basis

As before--for context, he'd typically used the more distant, but still respectful 'insurance-san' to refer to the girls, but Trigun vol2 (right before Fifth Moon) was the first time he ever said her name so seriously, which was a very Big Deal to her, and their last exchange for 2 years.

That upon their hospital reunion scene in Trimax vol3, genuinely happy and relieved but still awkwardly not quite knowing what to say to each other after all this time, she bonks him for trying his 'old' way of address--denied! No more of that please; say her name!

So aha, here in Trimax vol4, after Zazie captures Meryl as a 'weakness' to bait him and things get personal...there it is. Now he's quite direct and no-nonsense about it! Surprising Milly and shocking even his opponents (in a 'whoa wtf, he behaves nothing like Knives!' kind of way by jumping out of a window for her) with his immediate 'cut the bullshit; I don't care about your games, I'm only here for Meryl; give her back' >:[ attitude!

(For reference, Milly generally calls Meryl 'sempai' while Wolfwood calls everybody nicknames, referring to the girls by their big/small heights. So Vash asserting a First Name basis for her is pretty significant.)

And just to check if Vash continues using her name, on his Important Mission to find her, even all alone, no matter what (Trimax vol5 spoilers)...

Oh yes he does. (Face to face now.) She's that important to him.