Judge Juan Merchan has Donald Trump “under his thumb,” a former federal prosecutor said Thursday. Legal analyst Joyce Vance, who frequently comments on how the Trump criminal cases are being tried, highlighted the former president’s gag order hearing. Vance noted that the judge was “not impressed” with Trump’s team’s defense of his recent comments about the New York jury. ALSO READ: A criminologist explains how Americans achieve a post-Trump democracy “Trump commented that the ‘jury was picked so fast — 95% democrats. The area’s mostly all Democrat. You think of it as a — just a purely Democra…