Well, this poem's called "Upon Julia's Clothes," not "Upon Julia," so it's her clothes we should be paying attention to, right? And these clothes are strange. They're liquefying, glittering, vibrating, you name it. No wonder they're so compelling.
Line 1: The speaker starts to describe what happens when Julia wears silks. But this raises the question: would he be so interested in her if she weren't wearing silks? What happens when "in cottons my Julia goes"?
Lines 2-3: The Speaker describes the "liquefaction" of Julia's clothes. Clothes don't literally liquefy or flow. These words are metaphors the speaker uses to describe the dynamic movement of Julia's awesome outfit.
Line 5: Brave vibration is a metaphor which the Speaker uses to attempt to describe the movement of Julia's clothes. Since when do clothes vibrate? Is Julia dancing? These clothes seem to have a life of their own.
Line 6: It's hard to imagine clothes glittering, unless this is a costume on Dancing with the Stars. But we're thinking this Julia probably couldn't do a tango, so we should think of glittering as a metaphor to describe the movement of the clothes back and forth, which, given the right circumstances, might appear to shimmer and shine.