She's just afraid of the vac, which in and of itself just isn't a big deal. My female goes after the vac and is not a fearful dog and is a complete people junkie---no fear biting, issues etc.
You put your ankle in her path, and either she was just in a bit of a frenzy and didn't care what she bit at that point, or it was a redirect issue.
A redirect issue can be worked with, but as I'm sure you know, yelling or spanking isn't the cure and can just result in more issues.
When I vac, I use to tell my girl to "leave it," and if she did, I'd chuck a treat away from the vac. I no longer throw treats now that she gets the connection, and for the most part she ignores it . . . but attacking the vac is always on her mind, so I still do need to tell her to "leave it" now 'n then if she comes creeping around a corner or from under a table.