Currently the Vexmythoclast does not count towards primary weapon headshot bounties in the iron banner. I also encountered a glitch where the first shot had to charge. It was stuck like this until I switched my primary weapon to thorn and then back to vmc.
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These are both known issues, and the first has actually even been publicly addresed within [b][url=https://www.bungie.net/en/Help/Article/12702]this[/url][/b] article.