My oppinion
Meryl is looked on shallowly too often, which is a big mistake for such a deep character, but that is how Meryl-haters develop. They see her bossing others around, and they immediately dislike her without giving her a chance. PEOPLE, there is a reason for her behavior; you just have to look through her perspective! She has a job that may often require her to be harsh, and strict. If you're expecting a cold-blooded outlaw, and find a complete contrast, of course it would affect you drastically. It would have been such a contrast that it made her annoyed by Vash after creating this monstrous image of some heartless womanizer in her mind. Once she can except Vash, she realizes how much she cared for him.Meryl bashers are shallow -_-; Meryl's character is actually pretty realistic and like most Trigun characters, deep.
Her hold back would be never to tell Vash, because she is afraid if the answer, and maybe even afraid of her feelings.
I've got to love Meryl after analyzing her, and if those Meryl bashers did, they should at least appreciate her. She does add spice after all.
She also grows, and changes a lot during the series