Hi Al,
It does look suspiciously like a Telluric line. Compare for example Christian Buil's annotated spectra of Vega
http://astrosurf.com/buil/us/vatlas/vega3.gif
http://astrosurf.com/buil/us/vatlas/vega4.gif
from this page
http://astrosurf.com/buil/us/vatlas/vatlas.htm
Was the spectrum taken at a lower elevation compared with the others (higher air mass)?
Cheers
Robin