What is a vintail?

Vintail is an alternative spelling of ventail. Old French circa 14th century.

A ventail is a vent (go figure) to breath through on the front of a knight's helmet. Often it's a movable piece to allow the knight to salute and show his face as required without removing his helm. Another use of a ventail is a convenient place to tie on a lady's favor or other tokens of courtly love.

For my reference read a translation of Sir Gawain and the Green Knight, specifically the part where it describes all of Sir Gawain's armor.